Objective To observe the effects of constant light exposure on the obesity in high fat diet rats. Methods Thirty-two male SD rats were randomly divided into four groups : rats on a normal chow exposed to standard light-dark cycle (group A), rats on a normal chow exposed to constant light (group B), rats on a high fat diet exposed to standard light-dark cycle (group C), and rats on a high fat diet exposed to constant light (group D). Body weights and food intakes were recorded weekly throughout the 12-week study. Body weight, fat mass, visceral adipose tissue weight, intraperitoneal glucose tolerance test (IPGTT) results, insulin resistance parameters, serum lipids and levels of interleukin 6 (IL-6), tumor necrosis factor-α (TNF-α) were compared among groups. Epididymal adipose tissues mRNA expression of circadian clock genes, i. e. clock, bmal1, rorα, rev-erbα, cry1, per1, and per2 were analyzed by realtime PCR. Results From the 9th week, body weights of rats in group D were significantly higher than those in group C (all P<0.05). At the 12th week, area under curve of IPGTT (AUC-IPGTT) in groups B, C, and D were significantly higher than that in group A. AUC-IPGTT in group D was significantly higher than that in group C (all P<0.05). Compared with group C, a significant increase in fat mass, visceral adipose tissue weight, homeostasis model assessment for insulin resistance, serum cholesterol, TNF-α levels were observed in group D (all P<0.05). And a significant decrease in quantitative insulin sensitivity check index (QUICKI) and high density lipoprotein-cholesterol were observed in group D in comparison with group C (both P<0.05). Circadian clock genes (clock, rorα, rev-erbα, cry1, per1) mRNA expressions in group B and D were significantly different from those in group A (all P<0.05) . Expression of cry1 in group D was significantly higher than that in group C. In group C, rev-erbα mRNA expression was significantly down-regulated in comparison with group A (P<0.05). Conclusion Constant light exposure exaggerates obesity, glycolipid metabolism abnormality, inflammation, and insulin resistance in high fat diet rats. (Chin J Endocrinol Metab, 2017, 33: 1057-1062) Key words: Constant light exposure; High fat diet; Obesity; Insulin resistance; Circadian clock disruption