Yellow wine lees, the main co-products in yellow wine industry, consist of high content of crude protein but are not well utilized as animal feed. Solid-state fermentation was employed in the current study to upgrade the nutritional value of yellow wine lees as protein feed for ruminant animals. A single factor study was carried out to select the proper strain combination, and another single factor and a 3-level-3-factor orthogonal design were then conducted to evaluate the effects of the water content, incubation temperature and inoculum ratio on quality of product and find out the optimal condition for solid-state fermentation. Chemical composition and in vitro digestibility were determined to assess the nutritional value of the unfermented and fermented products. Each experiment was duplicated 3 times. The data were analyzed using PROC GLM of SAS. A strain combination of Candida and Bacillus was chosen for solid-state fermentation, and the optimal fermentation condition was water to total solid medium at 53:100 (v/w), 30°C of temperature, and ratio of Candida to Bacillus at 2:1. Contents (%, DM basis) of CP (30.2) and peptides (23.6) of fermented products were 14.5 (P < 0.01) and 40.9% (P < 0.01) higher than those of unfermented material, respectively. Content of AA of the fermented products increased by 25.7% (P < 0.01) compared with the unfermented, and contents of essential AA (such as Lys and Met) were increased greatly (P < 0.01). The microbial treatment decreased the ammonia-nitrogen concentration (P = 0.09) and increased the synthesis of microbial protein (P = 0.02) with higher crude protein digestibility (P < 0.01). The acetate content tended to decrease by the pretreatment (P = 0.08). These results drew the conclusion that the microbial pretreatment obtained from this study greatly increased the nutritional value of yellow wine lees, making these materials more suitable as feeds for animals including dairy cows.