Abstract: Multiaxial low cycle fatigue properties of titanium alloy BT9 and stainless steel 08X18H10T were investigated under proportional and non-proportional cycle loadings.Under non-proportional cycle loading, the additional hardening effect was obvious for 08X18H10T but not for BT9 and the fatigue life of both materials decreased dramatically.The TEM observation and analysis on the dislocation sub-structures of BT9 and 08X18H10T showed that the high dislocation density in some local areas of the dislocation sub-structures of BT9 and plastic deformation of 08X18H10 are major factors to aggravate LCF damage and shorten fatigue life.