The densification behavior of a 3-mol%-Y2O3-doped ZrO2 (3Y-ZrO2) has been investigated under N-2 and O-2 atmospheres. Powder compacts have been sintered at 1550degrees and 1400degreesC for various times. The density of the specimen sintered at 1550degreesC is higher in N-2 than in O-2, while the contrary result is obtained in the case of the specimen sintered at 1400degreesC. Such results can be explained in terms of nitrogen solubility and oxygen vacancy in a ZrO2 matrix. Because nitrogen solubility into the ZrO2 increases with an increase in heat-treatment temperature, leading to the formation of oxygen vacancy, the densification rate becomes higher. The present study thus shows evidence of nitrogen solubility into the ZrO2 and its role on the densification behavior of 3Y-ZrO2.