Abstract: The dissipation dynamic of fipronil in Job’ s-tears (Coix lacryma-jobi) seeds was studied,and the safe application was discussed. Results of field test showed that the half life of fipronil in Job’ s-tears seeds was 9.0~10.2 days (Taishun) and 10.8~11.3 days (Jinyun), respectively. The Job’ s-tears were treated two times with two dosages of fipronil during growing. It is 14 days (Taishun) and 7 days (Jinyun) that the fipronil residue was below 0.01 mg/kg in 30 g/hm2 treatment trials. Under the condition of 45 g/hm2 treatment, it is 30 days (Taishun) and 21 days (Jinyun) that the residue was less than 0.01 mg/kg in Job’ s-tears seeds. Therefore it is recommended that the pre-harvest interval (PHI) of fipronil in job’ s-tears is 30 days .