The heat transfer between rotor can/narrow air gap/silicon steel sheet has an important influence on the shrink fit process of reactor coolant pump rotor can. The accurate thermal conductance data are needed for the optimal design and numerical simulation of the shrink fit process of rotor can. Consequently, in this study, a set of new experimental apparatus and relevant procedure were developed to measure the thermal conductance at the interface between Hastelloy C-276/narrow air gap/silicon steel based on the transient method. The effects of the initial temperature of the Hastelloy C-276 specimen and the dimension of the air gap between the two test specimens on the thermal conductance were evaluated.