To analyze the true impact of fertilizer dosages used and the ending times of nitrogen fertilization before harvesting to vegetable yields and NO3 residues in pak choi crops, four dosages of nitrogen were applied: 70,80, 90 and 100 Kg N/ha to pak choi and 3 endings of nitrogen fertilizations before harvesting: 7 days, 10 days and 13 days. Main analysis indicators are the vegetable yield and concentration of NO3 accumulated in vegetables. Two statistical softwares: IRRISTAT, Minitab and analysis of variance models have been used for data processing and analysis of experimental results. Based on the ANOVA table for comparison and drawing comment, appropriate analysis of variance model with experimental design have been chosen. The results showed that the dosage of 80 Kg N/ha at 3 ending times of nitrogen fertilizations all offered higher yields compared with other nitrogen dosages, in which the ending of nitrogen fertilization at 10-days pre-harvest offered highest vegetable yield (48.66 tons/ha), the amount of NO3 residue was allow at safe level (449 mg/kg of fresh vegetables) in accordance with regulation of the World Health Organization (500 mg/kg of fresh vegetables).