An ultrasmall (<10 μm length) polarization converter in InP membrane is fabricated and characterized. The device relies on the beating between the two eigenmodes of chemically etched triangular waveguides. Measurements show a very high polarization conversion efficiency of >99% with insertion losses of <-1.2 dB at a wavelength of 1.53 μm. Furthermore, our design is found to be broadband and tolerant to dimension variations.