Unsteady and asymmetric behavior of separation of nominally axisymmetric and steady turbulent boundary layer in an axisymmetric expansion is investigated. The test diffusers have the total divergence angle from 8 to 50deg and length-to-diameter ratio of 8. Bulk Reynolds number is ranging between 2.5 × 104and 2.4 × 105. The wall static pressure fluctuation increases monotonically with increasing divergence angle up to 25° and then decreases. The wall static pressure exhibits large scale law frequency fluctuations, occurring almost concurrently in the entire passage. Distributions of both mean velocity and reverse flow fraction in a cross-stream plane show axial-asymmetry of the separated flow, suggesting the rapid growth of circumferential non-uniformity upstream. The velocity measurements employing a split film probe to distinguish the flow directions exhibits intermittent flow reversal with higher velocity as compared with forward flow. The conditional average of wall hot film signals reveals the steep decrease at the incipience of flow reversal.