This work examines the piloted ignition of a blended polypropylene fiberglass (PP/GL) composite material exposed to an external radiant flux in a forced convective flow of air. The effect of glass concentration on the ignition delay and critical heat flux for ignition at different external radiant fluxes and a fixed airflow velocity are determined experimentally. The effect of the airflow velocity on the ignition delay for a fixed fiberglass concentration is also stud-ied. The experiments are conducted in an apparatus (Forced Ignition and Spread Test) previously developed by the authors to study environmental effects on the piloted ignition of solid combustibles. The results of the study provide information about the effects of fiberglass concentration on the relative flammability of composite materials. It is found that the ignition delay and critical heat flux for ignition are functions of the fiberglass concentration in the composite, increasing as the glass concentration is increased. This is due primarily to the increase in the thermal inertia of the material by the fiberglass addition. The data are also used to develop an ignition diagram in terms of external heat flux and composite glass concentration that determines, for a given external heat flux, the critical concentration of fiberglass beyond which the PP/GL composite material does not ignite. This diagram is particularly useful for fire safety design purposes. The results for the effect of the flow velocity on the ignition delay show that the flow velocity affects the ignition delay particularly at values near the critical heat flux for ignition, increasing its value as the flow velocity is increased.