The tapered fiber as phase conjugator has the merits of high stimulated Brillouin scattering (SBS) reflectivity and high fidelity. In this paper, three self-designed tapered fibers with different dimensions were applied in a laser diode (LD) pumped high-power master oscillator power amplifier (MOPA) laser system operated at 100 Hz repetition-rate and 28 ns pulse width. Their SBS performances and influences of taper size on fiber phase-conjugator were experimentally studied. The results demonstrated that tapered fiber with core more than ?? 400 μm can be used in high-power laser system, while higher output energy and SBS reflectivity can be achieved by using a fiber phase conjugator with optimum taper size and larger fiber length of thinner side. Appling a 5.2 m-long tapered fiber with taper from ?? 400 μm down to ?? 200 μm, 85% energy reflectivity and 21 mJ output energy of double-pass can be obtained. The pulse width was compressed up to 17 ns, the peak power reached to MW.