To investigate the feasibility of eliminating the oil cooler in a hybrid electric vehicle（HEV）gasoline engine with a power density of 65 kW/L or above, a high-efficiency turbocharged engine specifically designed for HEVs was selected as the research object. Through experimental and simulation analysis methods, the engine operating conditions were analyzed. The changes in oil pressure, thermal balance, reliability, and fuel economy of both the engine and the vehicle before and after eliminating the oil cooler under the same operating conditions were compared. The research results show that, the cooling capacity of the system improves without the oil cooler. When the coolant temperature is 90 ℃, the wall temperatures of the cylinder head and cylinder block decrease by about 5 ℃ and 8 ℃, respectively. Meanwhile, the oil temperature rises, oil viscosity decreases, and the specific fuel consumption of the engine can be reduced by more than 0.8%. Under world light vehicle test cycle（WLTC）conditions, after eliminating the oil cooler, the oil temperature rise rate during the warm-up process decreases by 13.8%, while the coolant temperature rise rate increases by 19.0%, and the fuel consumption of the vehicle increases by 0.29%. Eliminating the oil cooler presents a low risk of oil temperature exceeding limits or causing engine failure under HEV operating conditions, having minimal impact on overall vehicle fuel consumption while effectively reducing vehicle cost and enhancing market competitiveness. The engine dyno thermal balance test can evaluate safe operating conditions and boundaries for the engine without an oil cooler under HEV conditions and predict engine oil temperatures under certain HEV operating conditions, thereby effectively saving vehicle test resources.