Miscanthus is an energy crop with high potential for the biofuels industry, because of its high annual biomass yields, low nutrients and water requirements, and remarkable adaptability to different climate and soil conditions. Since the lignocellulosic structure of miscanthus is recalcitrant to digest, pretreatment is needed to enhance sugar release during the sequential enzymatic hydrolysis. Commonly used thermochemical pretreatment methods operate under harsh conditions and have a large water footprint. In contrast, fungal pretreatment can be performed under mild conditions, using minimal amounts of chemicals and water. However, a costly feedstock sterilization step is generally required prior to fungal pretreatment, in order to eliminate negative impacts of indigenous microorganisms. Using pre-colonized feedstock as the inoculum is a promising strategy to address this issue, but has not been studied in fungal pretreatment of grassy biomass, such as miscanthus. In this study, fungal pretreatment of non-sterile Miscanthus × giganteous with Ceriporiopsis subvermispora was conducted, using previously colonized miscanthus as an inoculum. Five different inoculum ratios (10, 20, 30, 40, and 50%), and two moisture contents (60 and 75%) were tested. Results showed that inoculum ratios equal or greater than 30% enhanced the enzymatic digestibility of miscanthus 3-4-fold, which was consistent with the lignin degradation during pretreatment. Sugar loss up to 24% during fungal pretreatment was observed, but the final sugar yield could still be improved 2.5-4-fold.