A radiator grille tester of the present invention includes two adapters, a transparent tube, and a detector. The transparent tube and the detector are located between the adapters. The adapters, the transparent tube, and the detector are serial connected and define a channel. The adapters can be connected to an engine and a radiator respectively. Therefore, water in the engine and the radiator can flow passing through the transparent tube and the detector via the adapter. User can observe the water flow directly so as to check whether the engine and the radiator are malfunctioned. Observation and examination procedures are simplified.