Global warming has the potential to increase air temperatures by 1.8 to 4.0°C by the end of the 21st century. In order to reveal the effects of increased temperatures on the sucrose metabolism and cellulose synthesis in cotton fibre during its flowering and boll formation stage, field experiments with elevated temperature regimes (32.6/28.6°C, mean daytime/night-time temperature during flowering and boll formation stage during 2010-12, the same below) and ambient temperature regimes (30.1/25.8°C) were conducted. Activities of sucrose synthase and acid/alkaline invertase decreased under elevated temperature in fibre, but activities of sucrose phosphate synthase were increased. Callose content increased, but sucrose content decreased within the cotton fibre under elevated temperature. The disparity of callose content and sucrose content between the two temperature regimes decreased with the number of days post anthesis, indicating that the effects of elevated temperature on both sucrose content and cellulose content were diminished as the boll matured. Due to the dynamics of the carbohydrate content and associated enzyme activities, we hypothesise that the restrained sucrose metabolism and cellulose biosynthesis under elevated temperatures were mainly attributed to the changed activities of sucrose synthase and invertase. Furthermore, 32.6/28.6°C had a negative effect on the cellulose synthesis compared with 30.1/25.8°C.