This study aimed to investigate the acute effects of oleic acid (OA) on glucose homeostasis in mice fed a standard chow diet (SCD) and a high-fructose, high-fat diet (HFrHFD); moreover, the role of free fatty acid receptor 1 (FFAR1) was evaluated. The mice in the two groups were further divided into three subgroups as follows: control, OA (40 mg/kg), and OA + GW1100 (0.4 mg/kg, selective FFAR1 blocker). After a 16-week feeding period, the mice received the drugs via intraperitoneal (i.p.) injection followed by an i.p. glucose tolerance test (IPGTT) 30 min later. After 3 days, the mice received the same drugs to examine the effects of the drugs on the hepatic levels of phosphatidylinositol-4,5-bisphosphate (PIP2) and diacylglycerol (DAG). OA in the SCD-fed mice significantly increased the blood glucose level (48%, P < 0.001) after 30 min of glucose load compared to that in the control group, but did not affect the levels of PIP2 and DAG. Pre-injection with GW1100 significantly decreased the area under the curve of the IPGTT (28%, P < 0.05) in the SCD group compared to that in the SCD + OA group. OA reduced the blood glucose level (35%, P < 0.001) after 120 min of glucose load in the HFrHFD-fed mice; in addition, it increased hepatic PIP2 (160%, P < 0.01) and decreased hepatic DAG (60%, P < 0.001) levels. Pre-injection with GW1100 blocked the effects of OA on hepatic PIP2 and DAG without affecting the glucose tolerance. In conclusion, OA acutely impaired the glucose tolerance in the SCD-fed mice by acting on FFAR1 but did not improve it in the HFrHFD-fed mice.