A method and apparatus are disclosed from the perspective of a first device for performing sidelink transmission in a sidelink resource pool. In one embodiment, the first device has a configuration of the sidelink resource pool, wherein the sidelink resource pool is enabled with resource reservation for different Transport Blocks (TBs). The first device also has a configuration of a list of reserved periods. Furthermore, the first device selects or determines a first reserved period from the list of reserved periods, wherein the first selected or determined reserved period is within a first set of reserved periods. In addition, the first device randomly selects a first integer in a first interval, wherein the first interval is based on a scaling factor and a second interval, and the scaling factor is derived based on a largest reserved period in the first set of reserved periods, and wherein the first integer indicates a number of transmission opportunities of different TBs with the first reserved period. The first device further performs sidelink transmission of one TB on one transmission opportunity from the number of transmission opportunities.