Effects of ambient temperature on the ac electrical treeing and breakdown behaviors in an epoxy/layered silicate (1 wt%) were carried out in needle-plate electrode geometry. A layered silicate was exfoliated in an epoxy base resin,, using our ac electric field apparatus. To measure the treeing initiation and propagation, and the breakdown rate, constant alternating current (ac) of 10 kV (60 Hz) was applied to the specimen in a needle-plate electrode arrangement, at $30^{circ}C$, $90^{circ}C$ or $130^{circ}C$ of insulating oil bath. At $30^{circ}C$, the treeing initiation time and the breakdown time in the epoxy/layered silicate (1 wt%) system were 1.4 times higher than those of the neat epoxy resin. At $90^{circ}C$ (lower than Tg), electrical treeing was initiated in 55 min, and propagated until 1,390 min at the speed of $0.35{ imes}10^{-3}mm/min$, which was 4.4 times higher than that at $30^{circ}C$; however, there was almost no further treeing propagation after 1,390 min. At $130^{circ}C$ (higher than Tg), electrical treeing was initiated in 44 min, and propagated until 2,000 min at the speed of $0.96{ imes}10^{-3}mm/min$. Typical branch type electrical treeing was obtained from the neat epoxy and epoxy/layered silicate at $30^{circ}C$, while bush type treeing was observed out from the needle tip at $90^{circ}C$ and $130^{circ}C$.