Abstract: With 8\|year\|old Huangguogan in Shimian as materials，15N\|urea and urea were fertilized through two modes including the key phenophase （germination period，strong period of summer shoot，rapid enlargement of the fruit，veraison） fertilization and conventional （the falling period，strong period of summer shoot） fertilization. The difference in Ndff value in fruits and leaves during the strong periods of summer shoot，rapid enlargement of the fruit and veraison， and Ndff value，the distribution rate of 15N，the utilization rate of 15N in different parts of the tree during the maturity period were measured. The results showed that：in the three key periods of rapid enlargement of the fruit，veraison and maturity，the Ndff value of citrus fruit in the key phenophase fertilization mode was notable higher than that in conventional fertilization mode，the Ndff value of citrus leaves in veraison was notable lower than the other three phenophases in the two modes，which were 2.04 （the key phenophase fertilization） and 2.01（conventional fertilization） respectively. In the maturity period，the distribution rates of 15N were the highest in fruits both in the two modes，in which the distribution rate of 15N in fruits in the key phenophase fertilization mode was 502%，significantly higher than 45.7% in the conventional fertilization mode; Meanwhile， the utilization rate of 15N of citrus tree in the key phenophase fertilization mode was 35.7%，which was also significantly higher than 27.8% in the conventional fertilization mode. It was indicated that nitrogen could be absorbed and utilized much more fully by citrus trees in the key phenophase fertilization mode.

Key words: citrus；the key phenophase fertilization；15N\, urea；absorption；utilization