A bicycle crank assembly comprises a crank axle, a first crank arm, and a battery holding structure. The crank axle comprises a cavity and a first end portion. A battery unit is to be provided in the cavity. The first end portion has a first end opening which is in communication with the cavity. The first crank arm is configured to be mounted on the first end portion. The battery holding structure is configured to hold the battery unit in the cavity. The battery holding structure is configured so that the battery unit is replaceable through the first end opening in a state where the first crank arm is mounted on the first end portion of the crank axle.