Polarization-insensitive AlGaInAs-InP semiconductor optical amplifier is realized at wavelength of 1.55 μm. The active layer consists of three tensile-strained wells with a strain of 0.35%. The amplifier is fabricated with a ridge waveguide structure. The testing result shows that the amplifiers have an excellent polarization insensitivity (less than 0.5 dB) over the entire range of wavelength (from 1530 to 1580nm). The 1540 nm wavelength optical gain is 20 dB at the bias current of 200 mA. The AlGaInAs-InP optical amplifier shows good temperature characteristics, less than a 3dB reduction in the gain and polarization-insen sitivity when the temperature is raised from 25℃ to 65℃.