Optoacoustic interaction can be strongly enhanced in tiny core fibers, and it holds significant potential for stable harmonic mode-locking at gigahertz (GHz) and higher repetition rate. In this Letter, we propose and demonstrate a microfiber-assisted GHz harmonic mode-locking fiber laser, which is achieved by the enhanced optomechanical coupling between cavity modes in microfiber with the waist length of ∼16cm and the waist diameter of ∼1.56µm. The repetition rates can be stably locked at 2.3828 GHz and predominately locked at 1.7852 GHz, corresponding to the frequencies of radial R01 and torsional-radial TR21 acoustic modes, respectively. Our results provide novel insight into the design of a high-repetition-rate laser source and the application of microfibers in the optomechanical field.