The epoxy resin/polyurethane semi-IPN was prepared and the glass transition behavior of the semi-IPN was discussed with DSC and DMA methods. The results show that the two glass transition temperatures (T-g) referring to epoxy resin and polyurethane respectively get closer. Between the two T-g there exists another T-g related to the interface between the two polymers. SEM indicates that this semi-IPN has a two-phase continuous structure which changes with different weight compositions. This is also proved by testing the Young’s modulus. It is found that the IPN system has a cellular structure. Comparatively, the compatibility between the two polymers is the best when the weight ratio of EP/PU is 70/30.