The influence of Sn doping on improvement of minority carrier lifetime (MCL) of Fe contaminated directionally solidified p-type multi-crystalline Si ingots is studied. The macrostructure and resistivity distribution of the Si ingots indicate that no significant difference exists with and without Sn doping. The average MCL increase by 26.2%, 31.8%, 8.1% with 20 ppmw, 40 ppmw, 60 ppmw Sn doping, respectively. The MCL was improved evidently due to the reduction of formation of interstitial Fe, FeB. The doping of Sn promotes formation of vacancies, which also contributes to passivation of interstitial Fe and FeB. A calculation of constitutional supercooling is carried out, which shows that Fe have great influence on the solidification interface stability, and Sn have little influence on the interface stability.