Prediction of the fire hazard for a piece of garment exposed to a flaming heating source requires the development of a fabric ignition model which provides an accurate description of the flame-fabric interaction. Ignition times have been measured earlier on single fabrics by exposing them to a time invariant convective source. A theoretical correlation of these results has been carried out utilizing a new ignition criterion. For a porous, thermally thin material exposed to a normally impinging flame, quasi-steady conservation equations for species, energy and momentum have been formulated for the gas phase, and the transient effects have been accounted for through the variation of the fabric temperature with time. The predicted cellulosic fabric ignition times, under varied exposure conditions and including the effects of porosity, deviate from the measured ignition times by no more than 42%. Accounting for convective heat loss from the fabric improves the predicted results by about 5%. Porosity effects on ignition time are found to be small for the fabrics tested here. Pyrolysis appears to be the dominant process which controlls the ignition time.