The steels used for manufacturing nuclear reactor pressure vessel are low alloy ferritic steels. There is a range of temperatures at which these steels exhibit transition from ductile to brittle fracture known as the ductile to brittle transition temperature (DBTT). In these range of temperatures, ferritic steels exhibit a scatter in the fracture toughness values having a characteristic statistical distribution which is unique to ferritic steels. The master curve methodology aims to capture this behaviour of ferritic steels through fracture mechanics principles. To obtain master curve, fracture experiments of standard specimens as per ASTM E-1921 must be carried out. Since reactor pressure vessel steels suffer loss in ductility due to irradiation embrittlement during its service life, master curve needs to be generated at periodic intervals using surveillance specimens. Use of standard sized fracture specimens as surveillance specimens are difficult as the space available in a nuclear reactor for keeping these specimens are limited. Also, the radiation dose associated with testing standard sized irradiated specimens can be dangerous for the personnel involved. Under these circumstances, it becomes necessary to carry out such tests using miniaturized specimens. In this work, we will obtain the fracture toughness master curve of 20MnMoNi55 steel (low alloy ferritic steel used for making nuclear reactor pressure vessels) from miniature CT specimens having a thickness of 4 mm. © 2019 The Authors. Published by Elsevier B.V.