The thermal behavior, microstructure and crystal structure of 1,3-dioleoy1-2-palmitoyl glycerol (OPO); an additive demonstrated to improve the cold flow behavior of biodiesel, and methyl palmitate (MeP); a saturated FAME (Fatty acid methyl ester) implicated in the high melting temperature of common biodiesel, were investigated by DSC (Differential Scanning Calorimetry), PLM (polarized light microscopy) and XRD, respectively. Very complex and rich concentration dependent phase behavior was revealed attributed to specific intermolecular interactions between OPO and MeP. OPO delayed crystallization effectively and disrupted nucleation and growth altering crystal structure and microstructure profoundly. A steep drop in melting temperature accompanied a dramatic decrease of crystal size upon addition of OPO. A complete pseudo-equilibrium phase diagram of OPO/MeP including the thermal transitions below the liquidus line, polymorphism and microstructure development has been achieved. The study provides a comprehensive fundamental understanding that can help optimize the formulation of bio-sourced structured additives that would suppress crystallization and reduce crystal size of biodiesel effectively. (C) 2015 Elsevier Ltd. All rights reserved.