Objective: This study investigated the effect of different acute heat stress (HS) levels on chicken meat quality and ultra-structure. Methods: Chickens were randomly divided into 7 groups to receive different HS treatments: i) 36 degrees C for 1 h, ii) 36 degrees C for 2 h, iii) 38 degrees C for 1 h, iv) 38 degrees C for 2 h, v) 40 degrees C for 1 h, vi) 40 degrees C for 2 h, and vii) un-stressed control group (25 degrees C). Blood cortisol level, breasts initial temperature, color, pH, water holding capacity (WHC), protein solubility and ultra-structure were analyzed. Results: HS temperatures had significant effects on breast meat temperature, lightness (L-star), redness (a(star)), cooking loss and protein solubility (p<0.05). The HS at 36 degrees C increased L-24 h(star) value (p<0.01) and increased the cooking loss (p<0.05), but decreased a(24 h)(star) value (p0.05). Only the ultimate pH(24) (h) at 40 degrees C decreased compared to the control group (p<0.01). The pH in 36 degrees C group declined greater than other heat-stressed group in the first hour postmortem, which contributed breast muscle protein degeneration combining with high body temperature, and these variations reflected on poor meat quality parameters. The muscle fiber integrity level in group 40 degrees C was much better than those in 36 degrees C with the denatured position mainly focused on the interval of muscle fibers which probably contributes WHC and light reflection. Conclusion: HS at higher temperature (above 38 degrees C) before slaughter did not always lead to more pale and lower WHC breast meat. Breast meat quality parameters had a regression trend as HS temperature raised from 36 degrees C. The interval of muscle fibers at 24 h postmortem and greater pH decline rate with high body temperature in early postmortem period could be a reasonable explanation for the variation of meat quality parameters.