This work presents experimental and numerical results for the autoignition and pilot ignition of horizontal black polymethylmethacrylate (PMMA) plates in a cone calorimeter. Experimentally, the histories of plate surface temperature as well as ignition times have been measured for various radiative heating fluxes, Numerically, a two-dimensional axisymmetric model is proposed to simulate the transient processes in the gas and the solid plate. The radiative absorption by the gas phase monomer is considered in a simplified way. With a set of parameters for the one-step Arrhenius kinetics for the gas-phase reaction and a set of parameters for PMMA pyrolysis, the numerical results of surface temperatures and ignition times agree well with the experimental data for both autoignition and pilot ignition. Both experimental and numerical results indicate that while the pilot ignition is pyrolysis-controlled, the autoignition process depends not only on PMMA pyrolysis but also on gas-phase reaction, especially for lower heating fluxes. According to the numerical calculations for autoignition, the ignition times of thermally thin plates are significantly shorter than those of thermally thick plates. For pilot ignition, there exists a linear 1/roott(ig) - q(rad) relation for thermally thick plates, as indicated in the literature. (C) 2001 by The Combustion Institute.