This study aimed to evaluate the effectiveness of different conditions (i.e., different salt concentrations from 3.5 to 4.5%, and initial Lactobacillus plantarum populations from 10⁴ to 10⁶ CFU/mL) on the quality of fermentative grey oyster mushrooms at 15^oC during 25 days. The quality parameters i.e., total color difference, texture, total soluble solid, pH, lactic acid bacteria counts, total polyphenol content (TPC) and the free radical scavenging activity were measured during the fermentation. The results illustrated that the fermentation of grey oyster mushrooms would be in NaCl (3.5%) and initial Lactobacillus plantarum (10⁴ CFU/mL). The obtained data showed that although no significant difference was observed among different salt concentrations and inoculum starter culture on the quality of fermented mushrooms, the fermentation conditions enhanced the rising of TPC resulting in an increase in antioxidant activities of fermented oyster mushrooms. This study reveals that fermented grey oyster mushrooms can be used as a source of healthy plant-based foods by vegetarians to improve their nutrition diet.