Fracture toughness is an important material property to assess the critical load for structural integrity of reactor pressure vessel steel. In this paper, master curve method proposed by Kim Wallin is used to estimate the fracture toughness of 20MnMoNi55 steel in the ductile to brittle transition regime. Reference temperature (T₀) is evaluated using both single temperature and multi-temperature method for one inch thick compact tension (1T-CT) specimens. Reference temperature (T₀) is also determined from Charpy Vnotch test data and compared. Effect of selection of temperature range and number of test temperatures on the value of T₀ is also studied. It is observed that Charpy test results yield lower values of unirradiated T₀ compared to 1T-CT specimen tests. It is also observed that most of the fracture toughness values fall between 5% and 95% boundary of fracture toughness curves for all the evaluations.