In the present study, the process performance of anaerobic co-digestion of municipal sewage sludge and orange peel (OP) was evaluated. The experiment was conducted in batch mode under thermophilic conditions (55oC). It involved adding 1.5 and 3.0 g of OP to reactors R2 and R3, respectively. In R1 (control reactor), the mono-digestion of sewage sludge was conducted. The obtained results indicated that the application of OP led to deterioration of process efficiency. Decreased methane and biogas productions were noticed at both doses of OP. The average values of methane production were 518.9, 416.8 and 458.6 mLCH4 g-1 VS in R1, R2 and R3, respectively. The declining tendency on the biogas and methane rates was also observed. The application of OP resulted in degradation of the stability parameters. The negative effect of OP application was related with the presence of inhibitors such as ammonia nitrogen, volatile fatty acids, limonene and phenol, importantly theirs increased contents were observed in R2 and R3. Moreover, the thermophilic conditions are not recommended for the anaerobic co-digestion of those substrates, because they might accelerate the inhibition phenomenon.