In the study fracture toughness tests were performed with 20% sidegrooved small (10*10*55 mm) and ultra-small (3*4*27,5*5*27 and 5*10*55 mm) specimens. The objective of the study was to experimentally determine the measuring capacity of small specimens and to check the data against the statistical cleavage fracture model developed by VTT. Two pressure vessel steels, an A533B base metal (HSST-3) and a VVER base metal of 15Kh2MFA type were used in the study. Most of the tests were performed in the transition range and cleavage fracture was observed in the tests. Additional tests were conducted in the temperature range of ductile material behavior. The data indicate that the T0 (100 MPa√m) reference temperature can be determined even with 3*4 mm specimens in consistency with larger specimens by applying only the statistical specimen size correction, without any consideration on specimen constraint. The fracture resistance curves measured with 5*10 mm specimens give a good technical estimation for the larger specimen R-curves.