Environmental stress cracking (ESC) in poly(methyl methaerylate) under different humidity conditions has been investigated. Constant stress-intensity factor (K) ring-type specimens were prepared, and all specimens were equilibrated at five different humidity conditions for about two years. ESC tests were carried out under the same humidity as specimens had been stored. Acoustic emission (AE) signals during ESC tests were also measured to examine the crack-growth behavior. The threshold K value (K-th) tended to increase with increasing humidity. At a relative humidity (RH) of 11%, crack growth occurred gradually until 40 ks under a K value of 0.70 MPam(1/2), and then the crack-growth rate began to increase and AE events were observed. A laser microscopic observation indicated that the crack extended by the coalescence between a main crack and a microcrack ahead of the main crack tip. AE signals generated are considered to be associated with the coalescence. At 98% RH, an incubation period where no crack growth was observed existed under a K value of 0.94 MPam(1/2), but the crack began to grow suddenly after that incubation period. This suggests that the craze at the crack tip may become weaker with increasing loading time under high humidity. Although the crack-growth rate at 98% RH was higher than that at 11% RH, no AE events were observed. This suggests that the crack extended stably in the craze at a crack tip, and sorbed water may make the craze growth easy. All the results suggest that two different ESC mechanisms are activated depending on sorbed water that are varied by humidity.