This study develops a damage mechanics-based crack growth model to predict crack length in a typical viscoelastic material (i.e. bitumen) under a rotational shear fatigue load. This crack growth model was derived using torque and dissipated strain energy equilibrium principles. The crack length was predicted using bitumen’s shear moduli and phase angles in the undamaged and damaged conditions, measured by linear amplitude sweep (LAS) tests and time sweep (TS) tests, respectively. The two tests were both performed using Dynamic Shear Rheometer (DSR), thus the crack growth model was named as a DSR-C model. To validate the DSR-C model, the crack lengths after the TS tests were measured using digital visualisation of cracking surfaces for one virgin bitumen and one polymer-modified bitumen at two temperatures (15, 20°C), two frequencies (10, 20 Hz) and two strain levels (5%, 7%) under unaged and aged conditions. Results show that the DSR-C model can accurately predict the crack length in the viscoelastic bitumen under the rotational shear fatigue load at different loading and material conditions. The crack growth includes initial transition period, steady growth period and rapid growth period under a controlled strain loading mode. The degradation of the material property results from the crack growth that initiates from the outer edge toward the centre of the sample under the rotational shear load.