: The similarity of carbon nanofibers with carbon nanotubes, and their unique structure, along with the potential applications of nanofibers had caused a great tendency towards researching on their synthesis through catalytic decomposition of different hydrocarbon sources. A great deal of research was done on the synthesis of carbon nanofibers. One of the most important goals was to find a production method which produces high quality product, has the potential to produce in large scales, and is economic. The objective of the present research was to synthesize carbon nanofibers and to find the optimum conditions for this process. The most important parameter in this regard was the diameter of the nanofiber. Larger diameter carbon nanofibers were considered better in many applications. In this thesis, catalyst vapor floating method was used. Catalyst floating method is one of the methods of chemical vapor deposition and has the capability of producing nanostructure in large amounts. It also has other advantages in comparison with other conventional methods including low cost and continuous process. Different materials could be used in synthesizing carbon nanofibers. In the present work, considering the literature, xylene was selected as the carbon source, ferrocene as the catalyst, and hydrogen as the carrier gas which contributes to a reductive environment. Thiophene was also used in this process because of its effective role in nanofiber formation which was due to the presence of sulfur in its combination. Sulfur shifted the melting point above the eutectic point, which impoved the synthesis ofnanofibers considerably. The experiments weredone by altering the reactor diameter, hydrogen flow rate, and the weight percentages of ferrocene and thiophene. Considering the limitation in the number of reactors with different diameters, this parameter was not taken into consideration in the experimental design and only the comparison was made between the samples produced in two quarts reactors of differents diameters but in exactly the same conditions. In order to make meaningful tests and optimize the operational conditions, Minitab 16 software was used. After carrying out a number of experiments considering hydrogen flow rate, temperature, catalyst concentration, and thiopheneconcentartion as the parametes of designing experiments, hydrogen flow rate was omitted from the experiment design and another experiment design was done for the other three parameters. The reason referred to the bad results with low hydrogen flow rate along with the limitation of the present hydrogen generator. In new set of experiments, hydrogen flow rate was considered as the constant flow of 320 ml/ min. The experiment design was done considering the limits suggested for each of the parameters and in five levels. Minitab 16 recieved the quantitative data from the experiments (mean diameter of nanofibers) and presented a second order model of the three parameters for synthesizing nanofibers. Using the obtained model, the optimized conditions were determined as the temperature of 1042?C, thiophene concentration of 2 g/ml, and ferrocene concentration of 0.164 g/ml. Then, the synthesis process was carried out according the optimum conditions suggested by the model and the sample was tested through scanning electron microscopy, X-Ray diffraction, transition electron microscopy, and Fourier transform Infra-red. The analysis results of the test showed a good agreement with the experimental results and confirmed the prediction of the model. In the present work, the influence of each studied parameter on the synthesis of carbon nanofiberswas investigated and described in details. Carrying out this project, we came to the conclusion that increasing hydrogen flow rate would contribute to the synthesis of nanofibers with larger diameters. Increasing the temperature up to a limit, had the same effect. However, in higher temperatures, increasing the temperature contributed to the synthesis of smaller nanofibers. Another result was that increasing the catalyst concentration caused an increase in the diameter of the produced carbon nanofibers. The main conclusion of the research was the capability of the model presented, in controlling and also investigation of the conditions in which carbon nanofibers were synthesized. Keywords : carbon nanofibers, chemical vapor deposition, catalyst floating, ferrocene, Minita