Ionic liquids have high potential as novel lubricants because of their unique physical properties. In particular, halogen-free ionic liquids are low environmental-load lubricants. Unfortunately, these ionic liquids exhibit a poor tribological performance against bearing steel. However, they exhibit high performances against diamondlike carbon (DLC) under the high vacuum condition. The influence of DLC species on tribological performances and the reaction mechanism of halogen-free ionic liquids on the worn surface are still unknown. This investigation evaluates the tribological performances of halogen-free ionic liquids (1-ethyl-3-methylimidazolium dicyanamide ([EMIM][DCN]), 1-ethyl-3-methylimidazolium tricyanomethane ([EMIM][TCC]), and 1-ethyl-3-methylimidazolium tetracyanoborate ([EMIM][TCB])) against two types of DLC (tetrahedral amorphous carbon (ta-C) and hydrogenated amorphous carbon (a-C:H)). The tribological performances of ionic liquids differed by the anion structure and the type of DLCs. The DLCs lubricated with [EMIM][DCN] and ta-C lubricated with [EMIM][TCC] exhibited a low friction coefficient. On the other hand, other ionic liquids exhibited the high friction coefficients against DLCs. A time-of-flight secondary-ion mass spectrometer analysis indicated that the tribo-decomposition of halogen-free ionic liquids and the adsorption of anion achieve a low friction. The chemical activity of nascent surfaces plays a very important role in achieving tribo-decomposition. However, because this activity of DLC was lower than that of bearing steel, [EMIM][TCC] exhibited a poor tribological performance. Thus, although the tribological performances of halogen-free ionic liquids can be improved by using DLC as sliding materials, there is also the possibility of inhibiting the formation of an adsorption layer derived from ionic liquids.