Polydimethylsiloxane/polymethylacrylate(PDMS/PMA) interpenetratrating polymer networks(IPNs) was fabricated by simultaneous synthesis and characterized by thermogravimetry (TG). The kinetics of the thermal decomposition of polysiloxane/polymethylacrylate IPNs in N2 and air was studied at different heating rates. It was observed that there were two decomposition peaks both in N2 and air, which showed that thermal decomposition process obeyed two-step pyrolysis mechanism and accorded with the two compositions of IPN. Kinetic parameters were determined during simultaneous integration of the Doyle equation and the IPN/s kinetics equation was one-order equation in N2 and Air. The decomposition activation energy in N2 and air were 147.405 kJ·monl-1 and 192.656 kJ·monl-1 respectively.