Polymorphic crystallization of SOS (sn-l,3-distearoyl-2-oleoyl glycerol) has been studied with a time-resolved synchrotron radiation X-ray diffraction method (SR-XRD) using pure samples. SOS has five polymorphs : alpha, gamma, beta’, beta(2), and beta(1). Emphasis was placed on analyzing the events of melt-mediated transformation, in which the crystallization of more stable forms (gamma, beta’, and beta(2)) was induced by rapid melting of the less stable forms. This mode of crystallization is closely related to a long running dispute concerning a "memory effect" possibly occurring in the polymorphic crystallization of triacylglycerols from liquid. Two types of thermal processes were applied : (a) a rapid heating soon after the least stable alpha form was crystallized from liquid and (b) a rapid heating after thermal annealing of the alpha form for 20 min before its melting. The SR-XRD measurements were monitored at a time interval of IO s. The main results are as follows : (1) the formation of lamellar ordering of SOS occurred more rapidly than that of subcell packing, as exhibited in the earlier occurrence of SR-XRD long spacing spectra in comparison to the short spacing spectra, (2) the alpha-melt mediation without thermal annealing unveiled the formation of two types of liquid crystalline structures having long spacing values of 5.1 and 4.6 nm, (3) the gamma form directly occurred in the alpha-melt mediation involving the thermal annealing, which generated gamma-embryos in the solid phase of alpha to serve as seed materials, and (4) the formation of the second most stable beta(2) form of SOS was obtained after quite long incubation of the beta’-melt mediation, yet the most stable beta(1) did not crystallize. The present study has given deeper insights of the polymorphism of SOS in comparison to previous studies with optical, calorimetric, and conventional XRD techniques : in particular the presence of the liquid crystalline form and the formation of gamma-embryos were directly observed for the first time.