Optimum pre-reducing agents used to convert Se(VI) to Se(IV) for speciation analysis of inorganic selenium in tomato were investigated by FI-HGAAS. It was found that optimal conditions of Se(IV) consisted of 2 M HCl as carrier and 0.3%(w/v) NaBH 4 in 0.2%(w/v) NaOH as reducing agent. Concerning the completely pre-reduction of Se(VI), various reducing agents were studied including KCl, KBr, KI, ascorbic acid and thiourea in association with dilute and concentrate HCl and effect of temperature. From results, two optimal pre-reducing agents:0.5%(w/v) KI/conc. HCl and 0.5%(w/v) thiourea/conc. HCl and heating for 30 min were chosen, giving a linearity between 5 and 25 ng/mL. LOD and LOQ were 0.5 and 2 ng/L, respectively. Both of pre-reducing agents were then applied to tomato digest based on dry basis of its sample, which comparatively resulted in 5.10 mg Se(IV)/kg and 29.12 mg Se(VI)/kg using 0.5%(w/v) KI/conc. HCl, and 3.84 mg Se(IV)/ kg and 22.29 mg Se(VI)/kg using 0.5%(w/v) thiourea/conc. HCl.