The formula of ball piston pair's leakage in ball piston pump was gained under the condition of variable viscosity when ball piston was eccentric with cylinder block. The result has been compared with the one that got under the condition of supposing oil viscosity be constant. The conclusion was educed, with which the leakage of ball piston pair depended not only on clearance between ball piston and rotary cylinder block, pressure of working oil and speed of cylinder block, but also on the viscosity-temperature coefficient, viscosity-pressure coefficient, density and specific heat capacity of working oil. The influence of those factors on leakage of ball piston pair has been analyzed.