An oil flow control structure for grease gun has a hollow main body. An inflow end and an outflow end are respectively disposed at two sides of the main body. A high pressure passage and a low pressure passage, in which its inner diameter is larger than the high pressure passage, are disposed between the inflow end and the outflow end. A first pumping rod and a second pumping rod are respectively disposed in the two passages. The first pumping rod is pivoted to a pressing stem. The second pumping rod is pivoted to the pressing stem through a switching mechanism. The switching mechanism can regulate a pivot position of the second pumping rod relative to the pressing stem so that an initial position of the second pumping rod performing piston motion in the low pressure passage can be changed to seal or open the second inflow port.