Abstract: Glass fiber-reinforced PA6 composites were prepared by melt-blending with a twin-screw extruder, and the effects of humidity, water absorption, nucleating agent and heat treatment on their dimensional stability were investigated. The results indicated that the hygroscopic expansion and internal stress release shrinkage affected the size of the composites, and the former was the dominant factor. The higher the ambient humidity, the longer was the time. As a result, the bigger was the size of the composites. Moreover, its size variation in the vertical flow direction was greater than that in the flow direction. The lower the environmental humidity, the longer was the time required to achieve the same water absorption rate, resulting in a larger size of the composites. With an increase of the content of nucleating agent, the size of the composites tended to decrease slightly. The size variation achieved a minimum value at a content of nucleating agent of 0.5 phr. However, the size changed little with further increasing the content of nucleating agent. The heat treatment promoted the internal stress release shrinkage of the composites, resulting in a decrease of size variation rate of the composites due to an opposite effect by hygroscopic expansion.