PET/cotton (65/35, 50/50, 40/60) blended fabrics were treated with tetrabromobisphenol A (TBBA)-epichlorohydrin (EPI) by a pad-batch method and/or tetrakis (hydroxy methyl) phosphonium chloride (THPC)-urea by a pad-batch method in an 1 or 2-batch process. Flame retardancy and synergism between Br and P compound were investigated through the LOI measurements. Durability of flame retardant was tested by laundering and flame-retardant mechanisms are also discussed based on thermogravimetric analysis. When fabrics were treated with TBBA-EPI or THPC-urea, both system improved the flame retardancy of blended fabrics. Though THPC-urea was more effective of the two, sufficient self-extinguishing property could not be obtained. When fabrics were treated with two kinds of flame retardant in a 2-bath system, Br-P synergism was observed. Durability of flame retardant was proved by laundering test. The flame-retardant mechanism of TBBA-EPI was confirmed as gas phase mechanism and that of THPC-urea was confirmed as condensed phase mechanism by thermogravimetric analysis.