Oyster mushroom (Pleurotus sajor-caju) is known to be highly nutritious food. However, the understanding of its quality and food products produced from it are still very limited. With the desire to increase the value of oyster mushroom and to create an intermediate product with high nutrients content for further processing, a cellulase enzyme-assisted extract of dried oyster mushroom was investigated. The effects of extraction conditions in terms of temperature (40, 45, 50, 55oC), pH (4.5, 5.0, 5.5, 6.0) and extraction time (4, 6, 8, 10 hrs) on nutrients and bioactive compounds of extract were studied. The results showed that the highest extraction efficiency was obtained at 50oC, pH 5.5 and 8 hrs of hydrolysis. The Brix of the extract was 2.53. In 100 g dry matter of extract, the content of saccharose, reducing sugar, total protein, amino acids, lysine, β-glucan, total phenolic and total flavonoid was 15.13 g, 19.41 g, 43.65 g, 7.96 g, 0.82 g, 10.63 mg, 62.60 mg TAE, 5.54 mg QE, respectively. The highest antioxidant capacity of the extract determined by Ferric reducing antioxidant power (FRAP assay) was 165.50 mM Fe2+/100 g dry matter and that measured by scavenging 2,2-diphenyl-1-picrylhydrazyl (DPPH) radicals was 69.32%.