It was attempted to estimate the fracture toughness transition curves of reactor pressure vessel (RPV) steels from the ball indentation and tensile test data using the indentation energy to fracture (IEF) model and the relationships describing the effect of stress state on fracture. In the IEF model the fracture toughness is expressed as a function of the ball indentation test parameters and critical mean contact pressure. From the relationships among the stress components the fracture stress is derived as a function of stress triaxiality and flow property. In this approach the fracture stress calculated for the stress triaxiality of indentation deformation is assumed as the critical mean contact pressure. Indentation and tensile tests were performed on six RPV steels at transition temperatures of -160-25 degrees C. The values of critical mean contact pressure were in the range 2500-2500 MPa, The temperature dependence of the estimated fracture toughness, K-JC , agreed well with that obtained by the master curve method of ASTM E 1921 using three-point bend (TPB) specimens. In addition, the critical fracture stresses were obtained by considering the stress triaxiality for the crack tip. All test materials revealed the values of the critical fracture stress ranging from 2100 to 2500 MPa. (C) 2000 Elsevier Science B.V. All rights reserved.