A series of novel organo-phosphorus flame retardants (FR) derived from 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) have been synthesized in order to investigate their flame retarding performances for acrylonitrile-butadiene-styrene copolymer (ABS). The success of synthesis was confirmed by 1H and 31P NMR. The flame retardancy performances evaluated by UL-94V test revealed that V-0 ratings are achieved when 27.5?30.0 wt% of FRs are added to the ABS/FR mixtures. The most dominating factors affecting the flame retardancy of phosphorus containing FR are the phosphorus (P) content in the mixture and the chemical structure of FR incorporated. It was found that 4.86?5.05 wt% P in the formulation is required to exhibit self-extinguishing ability when phosphinate or phophonate FRs are employed. On the other hand, no rating is obtained for phosphate type FR containing mixture even though higher amount of P, that is, 5.07 wt% is present in the mixture, indicating that oxidation state of the phosphorus element in FR is very important in governing the flame retardancy. The modes of actions of FRs in terms of gas versus condensed phase are also discussed. Thermogravimetric analysis (TGA) results showed that the FR containing compounds possess inferior thermal stability compared to neat ABS at the beginning of decomposition, suggesting that the thermal degradation of ABS is accelerated by the presence of FR at earlier stage of degradation and then hindered by the char formed at later stage of degradation.