In this paper, we study the effect of positronium formation on the gamma-ray spectra and energy deposition in Type Ia supernova models. Positronium is an exotic bound state of an electron and positron. Formation and its succesive decay into ortho/para positronium can effect the gamma-ray observables and energy deposition by radioactive isotopes in the supernova ejecta. The effect is more important in the late phase of supernova evolution.