In this study, the influence of the degree of crystallinity on the mechanical behaviour of semi-crystalline polyamide 6 is investigated. To describe the long-term and time-dependent mechanical response of polyamide 6, a phenomenological constitutive model of finite viscoelasticity is used. This model consists of a parallel arrangement of a nonlinear spring and multiple Maxwell elements, motivated by the 1D rheological generalized Maxwell model. To account for strain rate dependency, a nonlinear viscous evolution equation is proposed. This proposed evolution equation is incorporated into the viscoelasticity model in a thermodynamically consistent manner. On the experimental side, polyamide 6 was melt-mixed with amorphous cyclic olefin copolymer to produce blends representative of polyamide 6 with varying degrees of crystallinity (from 15% to 29%). Various isothermal tests, including monotonic tension, relaxation and cyclic tests at temperatures above the glass transition temperature, were performed on the blends to generate a material database. To obtain the viscoelastic behaviour over an extended period, the time–temperature superposition principle is used. The model parameters are identified through a staggered procedure using a nonlinear optimization method. Initial computations indicated that the model could reliably predict the viscoelastic strain rate-dependent loading and relaxation behaviour across different crystallinity levels and temperatures.