Modern seafloor hydrothermal process occurs in a variety of morphological, structural and petro- logical settings. Most of the hydrothermal systems are found along the mid-ocean ridges with basaltic rocks as their host rock. In recent years, as the ultramafic-rock hosted hydrothermal system is confirmed as a new type of hydrothermal system, comparative studies of the difference between the ultramafic rock- hosted hydrothermal system and the basalt-hosted hydrothermal system on the seafloor become an interna- tional hot spot. The formation of basaltic and ultramafic rocks is controlled by different geological factors. Therefore, to reveal the influences of these geological factors on the formation of the two kinds of hydro- thermal systems is of significance to the study of their origin, distribution and mineralization. From this point of view, this article made a review on previous studies about the geological settings, host rocks, heat source and fissure development of the basaltic and ultramafic rock-hosted hydrothermal systems along mid- ocean ridge areas. Our results indicate that the heat source and fissure development are controlled by the development stages of mid-ocean ridge volcanism and tectonism. In the stage of volcanism, it is dominated by basalt-hosted hydrothermal systems with the heat source from axial magma chamber; while in the tec- tonic stage, ultramafic rock-hosted hydrothermal systems dominates along the deep fissures.
