In this study, the effect of swirl injection on the combustion stability was investigated with experimental tests and numerical calculation. As a result, swirl injection seems to modify two main physical processes responsible for the occurrence of the LFI. With swirl injection, even the frequency characteristics of p′ and q′ of both 20Hz and 500Hz band were not changed. However, as the swirl intensity increases, the phase difference p′ and q′ in 500 Hz band showed an increase above π/2 showing a totally independent relation between p′ and q′. Therefore, it is believed that phase shift happened as the result of the complicate interaction of strong rotational flow structure near core region and the additional combustion in post chamber.