A semi-empirical model of mixed-mode sub-critical crack propagation is presented for orthotropic polymers subjected to continuous loading. The model considers a combination of opening and shearing propagation modes. Optical measurement of crack propagation in an orthotropic liquid crystalline polymer film for orientation angles, which define the orientation of the loading to the extrusion direction, of 45 and 90 degrees provides the empirical constants required by the model. The model is validated by comparison of predicted and measured crack propagation at 30, 60, and 75 degrees. The impact of orientation angle on crack propagation is significant. A critical angle is identified at which the crack propagation rates for the opening and shearing modes are equivalent. For orientation angles less than the critical angle the crack growth is dominated by the shearing mode. For orientation angles greater than the critical angle, the crack growth is dominated by the opening mode. For the liquid crystalline polymer film tested, the critical angle is 52.6 degrees. POLYM. ENG. SCI., 48:2216-2223, 2008. (C) 2008 Society of Plastics Engineers