Dietary intake of high trans-fatty acids (TFAs) is well known to increase the risk of cardiovascular diseases. However, few reports demonstrated definitive relationships between dietary TFAs and obesity. In addition, the difference in the gastrointestinal absorption rate of TFAs containing oil from that of cis-FAs containing oil was not taken into consideration in many rat studies. In experiment A, we investigated the difference in the apparent absorption rate of TFAs containing oil from control oil. Hydrogenated rapeseed oil and a mixture of camellia oil and tristearin (90:10 [w/w]) were used as TFA-containing test oil and control oil, respectively. Ten Wistar rats were divided into the control group or TFA group, and fed the respective diet containing the control oil or the test oil for 1 week. The apparent absorption rates of these oils were measured by fecal fat excretion rate and dietary fat intake. The results showed a significantly lower gastrointestinal apparent absorption rate of the test oil (93.1%) than that of the control oil (96.2%). In consideration of the apparent absorption rate of these dietary oils, the effects of dietary TFAs on body fat accumulation and energy metabolism were investigated in rats. Twenty-eight Wistar rats were divided into the control group or the TFA group. Each group received an isoenergetic diet containing the control oil or the test oil for 8 weeks. Pre- and postprandial metabolic rates were measured between weeks 7 and 8. The test oil-based diet did not significantly influence body weight gain, fat accumulation, and metabolic rate. In contrast, liver weight, hepatic triglyceride content, and serum non-high density lipoprotein (HDL)-cholesterol (CHO)/HDL-CHO ratio were significantly higher in the TFA group than in the control group. In conclusion, these findings suggest that dietary TFAs did not influence body fat accumulation but increased the levels of risk markers of cardiovascular diseases.