Abstract: Viscoplastic models in classical continua theory were unable to incorporate scale effect for granular materials due to the independence on scale. Cosserat continua theory with the internal length scale provides a possible approach to incorporate scale effect. A Perzyna’s viscoplastic model is developed based on Cosserat continua theory to investigate the scale effect on rheology for granular materials. The consistent operator in the framework of Cosserat continua theory is derived. The constitutive integration algorithm and its closed form of the consistent tangent modulus are also derived. The code is implemented by User-defined elements (UEL) in ABAQUS platform. Triaxial compression creep tests of soft rock, creep and stress relaxation tests of rockfill, are simulated by finite element method. The results using finite element analysis show good agreement with the experimental data in literatures, which verified the model. Meanwhile, sphericity index, roundness index, and mean diameter of the particle are adopted as measures of the internal length scale to incorporate the influence of particle size and shape on axial strain, deviator strain and deviator stress, showing that the pressure-sensitive and the scale effect on rheology for granular materials can be captured.