To provide a power transmission fluid composition suitable for use in extreme pressure.

The power transmission fluid composition contains (a) a base oil, (b) a phosphonic acid ester shown by the formula at an extreme pressure performance improving amount, a succinimide dispersant, and an additive composition containing optionally a metal-based detergent. The optional detergent substantially contains no calcium cation. In the formula, R1 represents about 8-24C hydrocarbyl group, and R2 and R3 are independently selected from hydrogen and about 1-8C hydrocarbyl group with a proviso R2 and R3 are not equally hydrogen.