The use of tris (2,3-dichloropropyl) phosphate (TCPP) as a flame retardant in epoxy resin cured by methltetraphydrophthalic anhydride was studied. Although TCPP is an organophosphorus flame retardant containing chlorine, the date obtained from LOI measurements showed that TCPP was not more effective compared to other flame retardants containing only phosphorus. In order to understand the mechanism of flame retardant action of TCPP on epoxy resin, the effect of TCPP on the thermal degradation of the resin was investigated using thermogravimetry, differential scanning calorimetry, infrared spectroscopy, and elemental analysis. TCPP was shown to act predominantly by condensed phase mechanism, to catalyze decomposition, and to enhance char formation. A mode of flame retardant action of TCPP on epoxy resin by phosphorus in condensed-phase was proposed.