The role of oxygen, cell killing threshold dose and efficacy improvement of anti-cancer via type-II phototherapy are analyzed by both analytic formulas and numerical data. Higher initial photoinitiator (PS) concentration (C 0 ) and/or light intensity (I 0 ) has a faster efficacy, due to faster depletion of PS and oxygen concentration. Without the external oxygen supply, higher C 0 or I 0 produces more efficient singlet oxygen, resulting a higher transient efficacy, but they reach the same steady-state value when oxygen is completely depleted. With the external oxygen supply, higher efficacy (with no steady-state limitation), or lower cell viability are achieved. The overall efficacy may be enhanced by resupply of PS or oxygen during the light exposure.