A new treatment of sodium void coefficient in the fast reactor transient analysis is proposed. The method is based on the multigroup exact perturbation theory and utilizes the void reactivity map constructed by the multigroup first order perturbation theory. The change in the spatial distribution of neutron flux is corrected by the flux ratio of the fluxes in the transient and the steady states calculated with few-energy groups. The void reactivities for a homogeneous and a heterogeneous reactor are calculated by the new method with 4 and 6 energy groups and are compared with standard values obtained by the 25 group exact perturbation calculation. It is demonstrated that the new method gives almost the same values as the standard values while the ordinary calculation with 4 and 6 groups gives considerable deviation from the standard calculation. The present method has been incorporated to the two-dimensional quasistatic transient analysis code QUASAR and the code has been applied to an analysis of a control rod withdrawal accident. The results are compared with those calculated by the ordinary quasi-static method.