In order to investigate the mechanical strength and deformation characteristic of alkali activated concrete(AAC) under complex stress state, uniaxial and biaxial compressive loading tests were conducted on normal weight aggregate AAC(NAAC) and light weight aggregate AAC(LAAC), compared with those of conventional cement concrete including normal weight concrete(NWC) and light weight concrete(LWC).The results indicate that the linear part at ascending branch of AAC specimens is longer, and the brittleness is greater than the conventional cement concrete counterpart. There is obvious difference in the micro mechanism of compressive failure of AAC and cement concrete. The strength envelope curves of AAC is approximately circular, while that of conventional cement concrete is a long ellipse. Based on the revised Kupfers strength criterion for conventional concrete and test results, a novel failure criterion is proposed for AAC materials under biaxial compression.