New forming processes should be capable of manufacturing complex and high quality components in addition to low production costs and high production speeds. The gas pressure forming process is one of the methods used in forming these parts. In this research, the formation of Al-A356 alloy sheet in incomplete cone with gas pressure process has been studied numerically and experimentally. In general, it can be said that the purpose of this study is to investigate the effect of parameters such as the ratio of diameter to height of the die, angle, temperature and pressure. In order to determine the effect of parameters, FILLING the corners of the die and distributing the thickness of the parts under different pressures has been investigated. Also, for simulating validation, the simulation results were first compared with experimental tests. The results show that the MOLD FILLING will improve around 15% by increasing the pressure from 1 bar to 8 bar. However, is reduced to less than 0. 5 mm. Although, the MOLD FILLING increased by decreasing of the ratio of diameter to height but the critical thickness of the sheet is decreased around 14%. It was also found that for having a proper MOLD FILLING, it is necessary to increase the pressure value around 25% when the cone angle decrease from 30º,to 0º, . The results also showed that by increasing the temperature from 350°, C to 550°, C, the required pressure for FILLING the MOLD decreased from 8 to 2 bar.