The high temperature deformation behavior of Fe-2Ni-1Cr-Mo steels was investigated by hot compression tests performed in the temperature range 900℃ to 1200℃ with the strain rates of 10^0/sec, 10^(-1)/sec, 10^(-2)/sec and 10^(-3)/sec. Flow curves for 3.18Mo added steel were explained by hyperbolic sine relationship, ε˙ = A(sinh(0.0146σ_P))^(4.46)exp(-578KJ/RT) due to the dual effects of strain hardening and dynamic recovery during deformation. The activation energy for hot deformation increased with increasing the Mo contents. The softening at high temperatures was mainly induced by dynamic recovery and dynamic recrystallization. Microstructural change after hot deformation was interpreted by Zener-Hollomon parameters(Z). The relationship between microstructure and flow behavior was explained.