Use of castor oil as a renewable polyol in the polyurethane foams has been creating an attractive research interest for many researchers since last 5 decades. In this article, we examine the structural stability of flexible polyurethane foam produced using castor oil-glycerol blend by complete replacement of synthetic polyol. Addition of castor oil in the foaming blend as a complete substitute of synthetic polyol results in instability of foam. However, addition of glycerol as a crosslinking agent in the blend helps in overcoming this instability. Cellular morphology, segmented phase morphology and bulk properties of foams were investigated using scanning electron microscope, Fourier transform infrared spectroscopy respectively. Castor oil-glycerol blend significantly improves the foaming process at low concentrations (till 15 wt % glycerol) whereas, at higher concentrations volumetric expanding liquid undergoes faster cross-linking leading to retarded foam growth (above 20 wt % glycerol). Strut thickness shows a sharp decline at 25 wt % glycerol. Polymer phase morphology shows absence of H-bonded urea resulting in discrete hard segmented morphology whereas urea domains undergo agglomeration without glycerol. Foams were also characterized for thermal and bulk mechanical properties. © 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim