Effects of microstructural and environmental factors on fatigue crack propagation behavior of welded regions of cast Ti-6Al-4V alloy were investigated. Fatigue crack propagation tests were conducted for the welded regions, which were processed by two different welding methods: gas tungsten arc (GTA) welding and electron beam (EB) welding, under various load ratios (R=O. 1, 0.9) and atmosphere (air, seawater). EB weld consisting of very thin alpha platelets had the faster crack propagation rate than the base metal and GTA weld, regardless of load ratio and atmosphere. Fatigue crack propagation rates at high load ratio (R=0.9) were faster than those obtained at low load ratio (R=0.1) and there was no crack closure at high load ratio (R=0.9), indicating that fatigue crack propagation at high load ratio was mainly controlled by intrinsic factors such as microstructure. Fatigue crack growth resistances in seawater atmosphere were slightly lower than those in air, but showed the similar trend with variation of specimen conditions. The degrees of crack closure were almost same regardless of specimen conditions in seawater atmosphere, suggesting that the fatigue crack propagation in seawater was mainly controlled by intrinsic factors such as microstructure.