The rice leaf folder, Cnaphalocrocis medinalis (Güenée), will frequently be under heat stress because of global warming. The young larvae would be liable to heat exposure owing to inability to make leaf fold for hiding themselves. Consequently, their heat tolerance might be closely related to population outbreak in summer. Therefore, the heat tolerance of the first-instar larvae was examined in this study. The results showed that the heat tolerance of young larvae was lower than that of the older ones. Survival rates of the first-instar larvae were more than 60% on exposure to 39°C for 5 h per day for 3 successive days, but it dropped to below 20% on exposure to 41°C for 5 h per day for 2 successive days. The first-instar larvae exposed to 39°C for less than 8 h per day for 1, 2, and 3 successive days exhibited similar rates of pupation and adult emergence, fecundity, and egg hatchability as the control. High temperature of 39°C influenced the first-instar larvae on a small scale, but 41°C influenced greatly. Young larvae of the rice leaf folder can tolerate short-term heat stress in the present summer, and population outbreaks might not be seriously restricted to global warming.