Three-pulse photon echo pea,k shift (3PEPS) measurement was applied to the investigation of the primary part (< 100 ps) of the solvation dynamics in a series of imidazolium ionic liquids (IL) with an organic dye, oxazinen 4 (Ox4), utilized as probe. The ultrafast solvent response in the range of <= 300 is exhibited dependence on the square root of the anion mass, indicating its relation with the inertial motion of anion. The inertial response of ILs with chloride anion was the fastest among other ILs with heavier and larger anions. Because Ox4 is a cationic dye, it holds a stronger interaction with the anion of IL, thus the ultrafast part of the solvation is strongly affected by the inertial motion of anions. The second solvation component in the range of <= 3.5 ps had better correlation with the reduced mass and the size of both ions included, indicating the beginning of a more global solvation process.