Creeping discharge characteristics in N2, SF6 and CO2 have been investigated under single pulse voltage application. Anomalous behaviour in creeping flashover voltage has been found in SF6/N2 and CO2/N2 mixtures. In SF6/N2, reduction of flashover voltage has been induced by small amount SF6 admixture in N2 not only for negative but also for positive by increase of front steepness of applied pulse. On the other hand, in CO2/N2 maximum value of flashover voltage has been observed for about 10~30%-CO2 admixture into N2. These anomalous phenomena should be associated with creeping corona development process and charge accumulation on dielectric barrier surface.