In recent years, potential induced degradation (PID) has been a topic of interest to scientists because of its significant impact on performance deterioration of solar cells. Research has shown that the major cause of PID defects for p-type Si solar cells is sodium (Na) ion migration from the soda-lime glass through the encapsulation layer into the solar cell. However, limited scientific investigations have been done on the influence and mechanism of Na ion decorated micro-cracks on the evolution of PID. In this work, Na ion decoration in micro-cracks as the important mechanism causing PID in p-type crystalline Si solar cells is presented and discussed in detail. Furthermore, a model of PID mechanism due to micro-cracks is proposed.