An oil pump rotor assembly comprising: an inner rotor having "n" external teeth ("n" is a natural number); and an outer rotor having (n+1) internal teeth which are engageable with the external teeth, wherein the oil pump rotor assembly is used in an oil pump which, during rotation of the inner and outer rotors, draws and discharges fluid by volume change of cells formed between the inner rotor and the outer rotor, wherein a clearance, which is defined between the teeth of the inner and outer rotors that together form one of the cells which has the minimum volume among the cells, is designated as "a", a clearance, which is defined between the teeth of the inner and outer rotors that together form one of the cells whose volume is increasing during rotation of the inner and outer rotors, is designated as "b", and a clearance, which is defined between the teeth of the inner and outer rotors that together form one of the cells which has the maximum volume among the cells, is designated as "c", characterised in that the following inequalities are satisfied: a ≤ b ≤ c, and a < c, and wherein when the clearance "b" of the cell positioned backward as viewed in the direction of rotation is further designated as "b1", and the clearance "b" in the cell positioned forward as viewed in the direction of rotation is further designated as "b2", the following inequality is satisfied: b ⁢ 1 ≤ b ⁢ 2, wherein a clearance, which is defined between the teeth of the inner and outer rotors that together form one of the cells, gradually and continuously increases as the cell rotationally moves from a position at which the volume of the cell is minimized to a position at which the volume of the cell is maximized. an inner rotor having "n" external teeth ("n" is a natural number); and an outer rotor having (n+1) internal teeth which are engageable with the external teeth, wherein the oil pump rotor assembly is used in an oil pump which, during rotation of the inner and outer rotors, draws and discharges fluid by volume change of cells formed between the inner rotor and the outer rotor, wherein a clearance, which is defined between the teeth of the inner and outer rotors that together form one of the cells which has the minimum volume among the cells, is designated as "a", a clearance, which is defined between the teeth of the inner and outer rotors that together form one of the cells whose volume is increasing during rotation of the inner and outer rotors, is designated as "b", and a clearance, which is defined between the teeth of the inner and outer rotors that together form one of the cells which has the maximum volume among the cells, is designated as "c", characterised in that the following inequalities are satisfied: a ≤ b ≤ c, and a < c, and wherein when the clearance "b" of the cell positioned backward as viewed in the direction of rotation is further designated as "b1", and the clearance "b" in the cell positioned forward as viewed in the direction of rotation is further designated as "b2", the following inequality is satisfied: b ⁢ 1 ≤ b ⁢ 2, wherein a clearance, which is defined between the teeth of the inner and outer rotors that together form one of the cells, gradually and continuously increases as the cell rotationally moves from a position at which the volume of the cell is minimized to a position at which the volume of the cell is maximized.