In this project, influence of geometrical parameters of the ceramic mold on microstructure and grain structure of GTD-111 Nickel-based superalloy from directional solidification was studied. For this purpose, two different designs of the stepped ceramic molds including cubic and cylindrical with change in cross section in two positions were designed and anufactured. Selected dimensions for samples were based upon changes in module ratio in different parts of GE-F5-B1 turbine blade consisting of root, platform, shroud, and shank. Directional solidification tests of the GTD-111 superalloys were conducted at different withdrawal velocities of 3, 6 and 9 mm / min. In addition, effect of ceramic mold thickness on grain and dendritic structures obtained from directional solidification was studied. Grain and dendritic structures of all samples were observed in both longitudinal and transverse cross-sections. Also, a coupled thermal-structural simulation model for directional solidification of the stepped specimen was developed. Validation of the simulation model was done using experimental data. Results indicated that number of grains decreased in all samples by increasing distance from the water-cooled copper chill, due to the competitive growth whose effects was increased for higher withdrawal rates. For a given withdrawal rate, the number of total grains in cubic sample was more than that in the cylindrical one, which is due to the higher surface areas of the cubic in comparison to the cylinderical. Characterizing dendritic structure of the stepped molds showed that cross-sectional changes did not affect dendrite arm spacing. However, the cubic sample had a finer dendritic structure than the cylinderical. By increasing distance from the water-cooled copper chill, secondary dendrite arm spacing (SDAS) did not change significantly, while primary dendrite arm spacing (PDAS) increased because of the reduction in thermal gradient. Results of this study showed that increasing mold thickening from 5 to 10 mm could increase width of the columnar grains, although dendrite coarsening was more sensible. Current research showed that stray grains were solidification defects occurring in sudden changes of cross sections both in cubic and cylinderical specimens. Increasing the platform size in cross-sectional changes and withdrawal rate can increase the possibility of stray grain formation. According to the experimental and simulation results, critical conditions for stray grain formation in terms of platform size, growth rate, and initial diameter were provided. Keywords: GTD-111 Nickel-Based Superalloy; Stray Grai Directional Solidification; Gas Turbine Blade