The solid lipid nanoparticles (SLNs) were prepared based on the phase behaviors of hot microemulsions which were quite different from normal microemulisons for the solid lipid as oil phase. The pseudoternary phase diagrams for the system Brij78/deoxycholic acid sodium (DAS)/ glycerol monostearate (GMS)/water with and without the addition of retinoic acid (RA) were obtained at 60 using home-made apparatus with temperature control. O/W and W/O region were all found in the phase diagram. The introduction of RA has increased the O/W microemulsion region greatly. GMS-SLN and RA-GMS-SLN were prepared by direct cooling of hot O/W microemulsion obtained according to the phase behaviors results. The mean particle size of GMS-SLN and RA-GMS-SLN investigated by PCS is about 10 nm. TEM images indicated that both SLNs were spherical particles with diameter about 10 nm. PCS results showed that the particle dimension was kept almost the same after three months, proving good stability of both SLNs. The study on the phase behaviors of drug-loading hot microemulsions has been proved to be very significative for the controllable preparation of SLNs which could be used as nanoscale drug delivery system (DDS) for water insoluble drugs.