We propose a photonic approach to generate triangular waveforms based on nonlinear polarization rotation (NPR) effect in a highly nonlinear fiber. A continuous wave probe light is polarization rotated by an intensity-modulated control light through the NPR effect. By tuning the cascaded polarization controller, the undesired second-order harmonic is well suppressed after photodetection. A 90-deg hybrid coupler is used to compensate the phase mismatch between the fundamental and third-order harmonic components, so that triangular waveforms are generated successfully. The proposed approach is theoretically analyzed and experimentally verified. Triangular waveforms at repetition rates of 3, 4, and 5 GHz with full-duty cycle are experimentally generated.