This work addresses an experimental investigation on the cleavage fracture behavior of an ASTM A285 Grade C pressure vessel steel. One purpose of this study is to enlarge previously reported work on mechanical and fracture properties for this class of steel to provide a more definite database for use in structural and defect analyses of pressurized components, including pressure vessels and storage tanks. Another purpose is to determine the reference temperature, T0, derived from the Master curve methodology which defines the dependence of fracture toughness with temperature for the tested material. Fracture toughness testing conducted on single edge bend SE(B) specimens in three-point loading extracted from an A285 Grade C pressure vessel steel plate provides the cleavage fracture resistance data in terms of the J-integral and crack tip opening displacement (CTOD) at cleavage instability, Jc and δc. Additional tensile and conventional Charpy tests produce further experimental data which serve to characterize the mechanical behavior of the tested pressure vessel steel. The experimental results reveal a strong effect of specimen geometry on Jc and δc-values associated with large scatter in the measured values of cleavage fracture toughness. Overall, the present investigation, when taken together with previous studies, provides a fairly extensive body of experimental results which describe in detail the fracture behavior of an ASTM A285 Grade C pressure vessel steel.