By the oxidation of liquid poly(1,2-butadiene) (LPB) with H2O2/HCOOH, epoxidate poly(1,2-butadiene) (ELPB) was obtained as a toughening agent to prepare diglycidyl ether bisphenol-A (DGEBA) epoxy composites by using V115 polyamide(PA) as a cross-linking agent. DGEBA, ELPB, and the composites were effectively Cured by PA at 100 degrees C for 2 h followed by postcuring at 170 degrees C for 1 h. Thermal gravimetric analysis results in air and nitrogen atmosphere showed that the thermal stability of composites could be improved by the addition of ELPB. Compared with DGEBA/PA, the composites exhibited a decrease in strength at yield but an increase in strain at break with the increase in ELPB amount. The composite with 10% ELPB exhibited both thermal stability and tenacity superior to those of DGEBA/PA and composites with 5 and 20%, ELPB, respectively. The improvements in thermal and mechanical properties of composites depended on the formation of Inter Penetrating Networks (IPN) among DGEBA/PA/ELPB and their distributions in the matrix. At an appropriate ELPB amount, the IPN, mostly made of DGEBA/PA/ELPB, may be distributed more evenly in the matrix; less ELPB resulted in the formation of IPN mainly, made of DGFBA/PA; excessive addition of ELPB resulted in the local aggregation of ELPB/PA and phase separations. The toughening mechanism was changed from chemically forming IPN made of DGEBA/PA/ELPB to physically reinforcing DGEBA/PA by ELPB/PA with the increase in ELPB addition. (C) 2009 Wiley Periodicals, Inc. J Appl Polym Sci 113:3165-3170,2009