In this paper the results of experimental and numerical basic investigations of the transonic flow and heat transfer characteristics in small-sized parallel and convergent nozzles at critical state are presented. The compressible laminar boundary layer equations with heat transfer are solved numerically using the finite difference method. The experimental results are in good agreement with the numerical ones, and the boundary curve between the laminar and turbulent regions is obtained. Furthermore, in order to verify the validity of this study, the two-dimensional critical flow in a nozzle is visualized by a holographic interferometer.