A TE-TM mode converter is proposed in a single trench GaInAsP/InP waveguide, which is fabricated by a single masking and etching process. Use of single-trench structure makes the design and the fabrication much simpler. The design of single-trench mode converter is described together with its fabrication in this article. We investigated the dependence of conversion efficiency on the waveguide width, trench depth, and trench position. Also, the wavelength dependence of mode conversion efficiency was calculated in a wavelength range between 1.5 microm to 1.58 microm. 95% TE-TM mode conversion was measured at a wavelength of 1.55 microm in a fabricated device with a 210-microm half-beat length.