Recent researches have already shown that graphene can be a very promising candidate for reducing friction and wear by being coated on various moving mechanical assemblies. Although its excellent lubricative properties have been examined in different environments experimentally, the effect of conditions on graphene tribology performance are not well-understood. This research demonstrates the friction and wear mechanisms of graphene at nanoscale by first-principles calculations performed under two different conditions: in vacuum and in hydrogen. Besides, this report has revealed the difference of tribological performance between graphene coating and graphite powder in vacuum. It is shown that the graphene coating has lower adhesion than graphite powder because of less dangling bonds. Furthermore, with the perfect lamellar structure and low surface energy, graphene sheets exhibits promising wear resistance. Additionally, the adhesion and wear resistance of graphene sheets can be improved by hydrogen-induced flexible electrons. The computational simulations show that the higher adhesion originates from hydrogen adsorption. It is really helpful for the ruptured graphene to extend the wear life by hydrogen passivation.