Abstract: Using 4embryogenesis cell lines(ECLs),which were the embryogenic suspensor mass(ESM)of the slash pine,critical factors affecting the maturation of somatic embryos(SEs)were identified and studied.The results showed that different ECLs and carbon sources in the medium were the key factors affecting the maturation of the SEs.The presence of PEG of different molecular weights in different concentrations with ABA and AC in the medium affected SEs both qualitatively and quantitatively.The most desirable medium for the embryo maturation was found to consist of LP+ABA70mg.L-1+PEG 8000 150g.L-1+maltose 60g.L-1+AC 0~2.0g.L-1+ agar 3.0g.L-1.On which,approximately 19.8mature cotyledonary embryos were harvested from 1cm diameter of ESM.