The long-term stressful utilization of forests and grasslands has led to ecosystem degradation and C loss. Since the late 1970s, China has launched six key national ecological restoration projects to protect its environment and restore degraded ecosystems. Here, we conducted a large-scale field investigation and a literature survey of biomass and soil C in China’s forest, shrubland, and grassland ecosystems across the regions where the six projects were implemented (∼16% of the country’s land area). We investigated the changes in the C stocks of these ecosystems to evaluate the contributions of the projects to the country’s C sink between 2001 and 2010. Over this decade, we estimated that the total annual C sink in the project region was 132 Tg C per y (1 Tg = 10<sup>12</sup> g), over half of which (74 Tg C per y, 56%) was attributed to the implementation of the projects. Our results demonstrate that these restoration projects have substantially contributed to CO<sub>2</sub> mitigation in China.