Azoisobutyronitrile is a well-known radical initiator for synthesis of many polymers. However, a fundamental and important property; diffusion constants (D) of azoisobutyronitrile and the radical have been missing. We successfully measured D of the photodecomposed radical in benzene within 10 mus time window by the laser-induced transient grating method and simultaneously D of azoisobutyronitrile by the Taylor dispersion method. D of the decomposed radical was smaller than that of azoisobutyronitrile and the slow motion is attributed to the enhanced radical-solvent intermolecular interaction.