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The Xinjiang Region is China's major concentrated cotton growing area. But nighttime temperature during the late growing season is a crucial factor affecting fiber quality, especially fiber bundle strength. The objective of this study was to determine the effects of nighttime temperature on fiber bundle strength and cellulose deposition via elevating nighttime temperature at different fiber development stages. The results indicated that elevated nighttime temperature from anthesis to the boll opening stage (N a) significantly advanced the time of cellulose deposition, the onset time and termination time of rapid cellulose deposition were all significant by 4.5 d in advance, and the deposition rate (V 1) significant was increased by 0.65%/d in the initial period from anthesis to the onset of rapid cellulose deposition. And these resulted in an increase of fiber bundle strength. When nighttime temperature was elevated either from anthesis to the onset of rapid cellulose deposition (N 1) or between the onset and termination of rapid cellulose deposition (N 2), it had a significantly shorter duration of rapid cellulose deposition by 3.6–5.7 d, and the deposition rate of rapid cellulose deposition significantly increased by 0.8–1.0%/d. Compared with N 1 , the V 1 was significantly reduced by 7.9% and the maximum cellulose content was increased by 7.3% in N 2. Fiber bundle strength was significantly increased at the N 2 , but significantly decreased by 1.3 cN/tex at the N 1. Elevated nighttime temperatures from the termination of rapid cellulose deposition to boll opening (N 3) had no significant effect on fiber bundle strength or cellulose deposition characteristics. In conclusion, the effects of nighttime temperature on fiber bundle strength mainly depended on the average cellulose deposition rate from anthesis to the onset of rapid cellulose deposition (V 1). • High night temperature between onset and termination times of rapid cellulose deposition made for higher fiber strength. • Elevated nighttime temperature from anthesis to onset time of rapid cellulose deposition resulted in poor fiber strength. • Effects of nighttime temperature on fiber bundle strength mainly depended on the average cellulose deposition rate (V 1). [ABSTRACT FROM AUTHOR]