(NH4)2MoO2F4 is a new effective anticarious agent. In order to ensure the safety in clinical application of (NH4)2MoO2F4, six toxicity tests were conducted. Results indicated that: (1) The LD50 was 203. 6 mg/kg after oral administration, which showed that the toxicity of (NH4)2MoO2F4 wasn't higher than that of silver ammounium fluoride which was used routinely. (2) After 90 days of oral administration, no toxicity reaction was found in the lowest dosage group (25 mg/kg), but gastric mucosa was found to have slight injury in the highest dosage group (100 mg/kg), which was conformable to the result of the cumulative toxicity test. (3) Injuries of mucosa, allergic reactions and pulp damages were not observed when (NH4)2MoO2F4 contacted with mucosa, skin and surface of dental crown. Therefore, clinical application of (NH4)2MoO2F4 is safe while using it in the stipulated dosage.