A simple, inexpensive, safe version of pyrolysis apparatus is developed base on the standard cone calorimeter (ASTM E 1354). A controllable oxygen concentration (0% to 21% by volume) environment in the vicinity of 80 mm by 80 mm square sample positioned under the cone radiant heater is achieved by means of "Controlled Atmosphere Pyrolysis Apparatus". Valid gasification mass loss rate measurements have been obtained for both poly(methyl methacrylate) (PMMA) and polypropylene (PP) samples under external heat fluxes of 35kW/m^2 and 50kW/m^2. Reasonable value of thermal conductivity for PMMA is measured. With the thermal conductivity and parameters defined by Differential Scanning Calorimeter (DSC) of PMMA, the gasification mass loss rate is well simulated using Thermo-Kinetic Model of Burning (ThermaKin).