Flame retardant rigid polyurethane foam (RPUF) with a reactive melamine-derived polyol (MADP) and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) were fabricated through free-rise technique. The thermal stability and fire behaviors of RPUF were investigated by thermogravimetric analysis (TGA), limiting oxygen index (LOI), and cone calorimeter. In comparison with the results of RPUF and RPUF/MADP, RPUF/MADP-DOPO displayed higher compressive strength, owing to the two stiff phenyl groups of DOPO. Moreover, with only 7.5 wt % MADP and 7.5 wt % DOPO loading, the MADP/DOPO system endowed RPUF with enhanced char formation, increased LOI value of 28.5% and reduced heat release. Scanning electron microscope (SEM) images, X-ray photoelectron spectroscopy (XPS) analysis and Raman spectra of the char residues suggested that the formation of compact, continuous char layer. The thermal degradation was characterized using thermogravimetric analysis infrared spectrometry (TG-IR) and chemical structure changes during the thermal degradation were monitored by real time Fourier transform infrared (RT-FTIR), these results demonstrated that the partial phosphorus from DOPO remained in the residual char, but the introduction of MADP significantly promoted generating more stable aromatic structure and enhanced the strength of the char layer formed. Furthermore, MADP facilitated the formation of more H2O and CO2 and diluted the flammable gases in gaseous phase. Based on the analysis, the introduction of MADP/DOPO can endowed excellent flame retardancy to RPUF depending on bi-phase flame-retardant mechanism. (C) 2018 Elsevier B.V. All rights reserved.