The effect of succinonitrile (SN) additive on cycle life at room temperature is performed with 18650 type cylindrical cell including Li(Ni0.5Co0.2Mn0.3)O2 and graphite. It is clearly investigated that the capacity degrades during cycling at room temperature with adding SN additive. In particular, the capacity of the cell with 1.5 wt % SN dramatically decreases from 60 cycles and by 45% of initial capacity after 100 cycles. Through analyzing cycled cells with different amount of SN additive using XRD and ICP-OES measurement, the amount of irreversible Li increases depending on the amount of SN additive and the number of cycling. The results of SEM and XPS measurement provide that the increased irreversible Li appears as a film-type reactant on the surface of anode electrode and it consists of components related with C-C, CO3, and Li. These analysis results verify that the SN additive affects generation of Li-carbonate species deposition and irreversible Li leads to a degradation of capacity.