Two-way shape memory effects (2 W-SMEs) have been recently demonstrated in several semicrystalline networks, the underlying mechanism of which is revealed as crystallization induced elongation (CIE) and melting induced contraction (MIC). In this article, a one-dimensional physically based constitutive model, considering the crystallization micro-mechanism at chain-scale, is proposed to describe the CIE and MIC phenomena, as well as the stress-strain-temperature relations of semicrystalline polymers in typical two-way shape memory cycles. The overall model is implemented into Mathematica and a quantitative evaluation of the model is performed by comparisons with the two-way shape memory behaviors of several kinds of semicrystalline networks. [ABSTRACT FROM AUTHOR]