The aim of the current study was to assess the effects of adding Chlorella vulgaris algae at different levels on in vitro gas production (GP) of three total mixed rations (TMR) with different concentrate (C): maize silage (S) ratios (25C : 75S, 50C : 50S, 75C : 25S). Chlorella vulgaris was added at 0, 20, 40 and 80 mg/g dry matter (DM) of the TMR and total gas, methane (CH4) and carbon dioxide (CO2) production were recorded after 2, 4, 6, 8, 10, 12, 24 and 48 h of incubation in three runs. Increasing concentrate portion in the TMR linearly increased the asymptotic GP and decreased the rate of GP without affecting the lag time. Addition of C. vulgaris at 20 mg/g DM to the 25C : 75S TMR increased the asymptotic GP, CH4, CO2 and GP at 48 h. Addition of C. vulgaris to the 50C : 50S TMR decreased the asymptotic GP and GP at 48 h. Higher CH4 production was observed at 48 h of incubation when C. vulgaris was included at (per g DM): 20 mg for the 25C : 75S ration, 40 mg for the 50C : 50S ration and 80 mg for the 75C : 25S ration. Inclusion of C. vulgaris linearly increased CH4 production for the 50C : 50S ration and increased CO2 production at 10 and 12 h of incubation for the 50C : 50S ration, whereas 20 and 40 mg C. vulgaris/g DM of the 75C : 25S TMR decreased CO2 production. The 25C : 75S TMR had the highest in vitro DM disappearance with C. vulgaris addition. Chlorella vulgaris addition was more effective with rations high in fibre content than those high in concentrates. It can be concluded that the optimal level of C. vulgaris addition was 20 mg/g DM for improved ruminal fermentation of the 25C : 75S TMR.