In this study, a three-dimensional micromechanics-based analytical model is developed to study the effects of regular and random distribution of silica nanoparticles on the thermo-elastic and viscoelastic properties of polymer nanocomposites. The Representative Volume Element (RVE) of nanocomposites consists of three phases including silica nanoparticles, polyimide matrix and interphase. Since the polymer in the vicinity of the nanoparticles shows distinct properties from those of the bulk matrix, because of nanoparticle–polymer matrix interactions, this region as interphase is considered in micromechanical modeling with specific thickness and properties. In order to simulate random distribution of silica nanoparticles into polyimide matrix, the RVE is extended to c×r×h cubic nano-cells in three dimensions. Perfect bonding conditions are applied between the constituents of RVE. It is assumed that all three phases of the RVE to be homogeneous and isotropic to obtain the thermo-elastic response of nanocomposite. The extracted thermo-elastic properties by the micromechanical model with random distribution of silica nanoparticles are closer to the experimental data. To predict the effective viscoelastic properties of the nanocomposites, silica nanoparticles are modeled as a linear elastic material, while polyimide matrix and interphase are assumed to be as a linear viscoelastic material. The model is also used to examine the influence of varying interphase properties and silica nanoparticle size on the effective nanocomposite behavior. The overall creep behavior of the nanocomposite for several stress levels is also presented.