Abstract: The contradiction between ecological environment and industrial economic development is extremely prominent in resource-based areas. We conducted an in-depth research on the effects and mechanisms of different types of industrial agglomeration on eco-efficiency in nine typical resource-based provinces (Shanxi, Inner Mongolia, Heilongjiang, Guizhou, Yunnan, Shaanxi, Qinghai, Ningxia and Xinjiang) from 2006 to 2015, including specialization, related and unrelated diversification. The results showed that the specialization had a negative impact on eco-efficiency. The industrial specialization degree of all provinces except Ningxia showed a downward trend, which was conducive to the improvement of eco-efficiency. The impacts of related diversification on eco-efficiency were characterized by a U-shaped curve. The degrees of related diversification in Shanxi, Qinghai and Ningxia were less than the critical value of 1.49, which inhibited the improvement of eco-efficiency. Other provinces had crossed the inflection point, with a promotion role of related diversification on eco-efficiency. The unrelated diversification had a negative impact on eco-efficiency. The degrees of unrelated diversification in Shanxi, Guizhou, Shaanxi, and Heilongjiang declined slightly, which was conducive to the improvement of eco-efficiency. In other provinces, the unrelated diversification increased significantly, which was not conducive to the improvement of eco-efficiency. Our results could provide theoretical basis for path selection and policy formulation of industrial green transformation in resource-based regions.