Wear in the contacting elements is of economical and industrial concern and has captured the attention of various researchers for several decades. A large number of publications have been devoted to the gears and bearings to enhance their performance and wear resistance. During running-in, which is the first part of the life of a pair of fresh surfaces in sliding contact, the friction coefficient and roughness profile of contacting elements experience changes. Thus, the operating conditions of running-in period play an important role in the steady-state performance of mechanical elements such as gears, cam-followers, bearings, etc. One of the methods to enhance the performance of mechanical elements is to nitride them. In this research, friction and wear of nitrided and un-nitrided disks are experimentally and numerically studied. The selected variables are load and speed and properties such as running-in length, friction coefficient changes, and weight loss are measured as the outputs. The experiments are conducted using pin on disk test rig on samples made from 4140 steel. For all of the experiments the SAE 10W40 oil was used as lubricant and the weight loss during the test was measured using a digital scale. For the sake of accuracy, after each experiment the disks were drowned in acetonebath using an ultrasonic mixer, to remove all the oil and debris. For nitriding the disks, a higher value of hardness was required. Thus the samples were quenched and tempered to a hardness value of 370 HV. Nitriding parameters were 15% nitrogen and 85% hydrogen in a temperature of 520 C and pressure of 5 torr for six hours. After nitriding, the hardness value of disks was 580HV. A model based on the load-sharing concept has been developed to predict the friction coefficient as well as the contribution of asperities and lubricant film in carrying the load. A simple analysis has been added to predict the surfaces and the lubricant temperature. The lubricant temperature is then employed to predict the wear volume using the fractional film defect theory. The results for un-nitrided disks indicate that increasing the speed results in a higher lubricant film thickness and thus the wear volume decreases. Increasing the applied load, on the other hand, results in a decrease in the film thickness and as a result the wear volume increases. In the nitride case, the wear volume is a function of load only and does not depend on the speed. The diagrams of wear volume versus sliding distance show that the running-in period finishes approximately before 1500m after which the wear rate stabilizes. The experimental results showed that the friction coefficient in the lubricated case does not change during time for the unnitrided disks nor for the nitrided disks. This result was repeated for both short distance and long distance tests. Comparison between un-nitrided and nitrided disks indicates that in small speeds wear in un-nitrated disks is four times larger than nitrided disks while in higher speeds this ratio decreases to two and half. Friction coefficient is 11-19% lower in nitrided disks than un-nitrided disks. Keywords: Running-in, lubrication, Nitrided, Friction coefficient, Wear, pin on disk.