Cyclin G1 is the only cyclin that has either positive or negative effects on cell growth. Our previous study found decreased expression of cyclin G1 in human endometrial carcinoma tissues compared with normal endometrial tissues. The study aimed to evaluate cyclin G1 expression and its effect on proliferation of human endometrial carcinoma cells (ECCs). Cyclin G1-GFP (green fluorescence protein) plasmid was constructed and transfected into various differentiated human ECCs, including Ishikawa, HEC-1-B and KLE cells, and proliferation of the transfected cells was determined by the CCK-8 method. Exogenous cyclin G1 mRNA and protein were measured by RT-PCR and Western blot, respectively, and GFP signal was monitored by fluorescence microscopy. Chinese hamster ovary (CHO) cells were transfected with the same constructs as a cell control. Cyclin G1-GFP-transfected Ishikawa cells were further treated with MG132, an inhibitor of proteasome, to analyze if low expression of cyclin G1 is related to its abnormal degradation in ECCs. Ectopic expression of exogenous cyclin G1 was found to significantly suppress the proliferation of Ishikawa and HEC-1-B cells but not KLE cells. Compared with cyclin G1-transfected CHO cells, exogenous cyclin G1 protein expression was low in Ishikawa and HEC-1-B cells, and was undetectable in KLE cells. However, all ECC lines and CHO cells expressed similar levels of exogenous cyclin G1 and GFP mRNA. MG132 treatment increased cyclin G1 protein expression in cyclin G1-GFP-transfected Ishikawa cells. This is the first study to present evidence to suggest that cyclin G1 exerts negative control on proliferation of ECCs. Exogenous cyclin G1 shows different protein expression levels in ECCs with different malignancies, and cyclin G1 protein is highly unstable and is rapidly degraded in ECCs.