The critical fracture stresses (σ* f(sp)) in various low alloy steels for a reactor pressure vessel(RPV) were evaluated by a small punch test (SP test) and a finite element analysis (FEA) in the cleavage temperature region(-150~-196 °C). The load-displacement curves and distances from the center to the fracture surface (Df) of the FEA results are in good agreement with the experimental results. The maximum principal stresses (SP fracture stresses, σf(SP)) were determined from the FE analysis, when the maximum load was applied to the SP test. The SP critical fracture stress, σ* f(sp) in various reactor pressure vessel (RPV) steels was found to have a linear relationship with the values obtained from the precracked specimens (σ* f(PCVN)). The σ* f(sp) shows a lower value than σ* f(PCVN) because the SP specimen had a lower triaxial stress condition. However, this result indicates that a small punch test could be a useful method to evaluate the cleavage fracture behavior of low alloy steels.