An experimental study was conducted to evaluate the effect of frequency and self-heating on fatigue behavior of two unreinforced and two short glass fiber reinforced thermoplastic polymers. Load-controlled fatigue tests were conducted under fully reversed (R = -1) and R = 0.1 conditions with specimens loaded in either longitudinal or transverse direction to the mold flow direction. Effect of frequency on fatigue life was evaluated at 23 and 125 degrees C and for a range of frequencies between 0.063 and 20 Hz. Incremental step frequency tests were also performed at different stress ratios and stress levels. Surface temperature rise was found to be material, frequency, and stress level dependent. Three energy-based models were applied to the incremental step frequency data and relationships were developed for each material to estimate surface temperature rise as a function of test frequency and stress level. Relationships were also developed to assess critical frequency for the unreinforced thermoplastics at a given stress level above which surface temperature does not stabilize. (C) 2015 Society of Plastics Engineers