Abstract: In VSAT satellite communication system using slotted ALOHA method, assigning guard time is important for frame format design. In this paper, sufficient and necessary condition for collision-free receiving in quasi synchronous ALOHA system was reduced from timing relation. Based on this conclusion, the joint effects of guard time and timing warp on system performance were analyzed and a scheme was proposed to calculate optimum guard coefficient. Numeric simulations show that when timing warp coefficient月镇0. 5，there is a optimum guard coefficient app，and using it a guard time achieving maximal throughput can be obtain. When timing warp coefficient月0. 5，increasing frame guard time will decrease system throughput.

Key words: VSAh, Quasi-synchronous, Slotted ALOHA,hhroughput