Rigid polyurethane foam (PUF) filled with Mixture of alumina trihydrate (ATH) and triphenyl phosphate (TPP) as fire retardant additive was prepared with water as a blowing agent. In this study, the ATH content was varied from 10 to 100 parts per hundred polyol by weight (php), and TPP was used at a higher loading of ATH (75 and 100 php) in a ratio of 1 : 5 to enhance the processing during PUF preparation. The effects of ATH on properties such as density, compressive strength, morphological, thermal conductivity, thermal stability, flame-retardant (FR) behavior, and smoke characteristics were studied. The density and compressive strength of the ATH-filled PUF decreased initially and then increased with further increase in ATH content. There was no significant change in the thermal stability with increasing ATH loading. We determined the FR properties of these foam samples by measuring the limiting oxygen index (LOI), smoke density., rate of burning, and char yield. The addition of ATH with TPP to PUF significantly decreased the flame-spread rate and increased LOI The addition of TPP resulted in easy processing and also improved M characteristics of the foam. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 116: 2260-2268, 2010