Using an optical interferometric technique, the grease film distribution under a pure rolling reciprocating motion is observed on a ball-disk test rig, to explore the transient response of grease, which is expected to fill the contact with thickener fiber and run for a long time under steady-state conditions. It is found that the reciprocating motion reduces the accumulation of the thickener fiber gradually with time. The maximum film thickness forms around the stroke ends. After 1,000 working cycles, very severe starvation occurs so that either the central or minimum film thickness becomes nearly constant over one working period. Thus, the life of grease lubrication under a transient condition is far below that under steady-state conditions. However, it is also found that by selecting a smaller stroke length, the thickener fiber spreads out in the contact instead of being removed from the contact at the 1,000th working period. When increasing the maximum entraining speed of the reciprocating motion to a certain value, during which the thickener fiber is not expected to accumulate under a steady-state condition, severe starvation occurs very quickly, causing surface damage.