The role of water in the material removal of single crystal calcium fluoride (CaF2) was studied using a scratch tester with a diamond tip. Water can facilitate the sliding wear of CaF2, and the degree strongly depended on the amount of water molecules involved in the wear process. CaF2 wear increases with the increase of relative humidity (RH). When the sliding wear tests were conducted in dry condition, the wear rate increases linearly with applied load, obeying the Archard wear equation. When water participated in the wear process, the Archard wear equation was failure and a exponential dependence of wear rate on contact stress was given under water condition. The transformation of the applied load (or contact stress) dependence indicates the change of wear mechanism in water condition. In dry condition, the surface damage of CaF2 is mainly attributed to mechanical wear where the stress-induced plastic flow under the activation of the slip systems results in a relatively smooth worn surface. In humid nitrogen or water condition, the plastic wear of CaF2 weakens due to the stress-associated dissolution though the wear volume increases compared to that in dry case. Both the water-associated dissolution itself and induced delamination promote the sliding wear of CaF2 in water. The results may help elucidate the water-associated wear mechanism of CaF2 and optimize the processing parameters in the ultra-precision machining to reduce the residual surface/subsurface defects.