Conversion of animal waste to biogas energy to replace traditional fuel and use of the slurry as a fertilizer is the current focus of the national biogas program of Ethiopia (NBPE). However, there are many plant species which could also be probed for their potential of biogas production. This paper presents the experimental results of the anaerobic digestion of Justicia schimperiana (JS) and cow dung each separately and with their various combinations at Addis Ababa University Environmental Science Laboratory. The biomass of JS and cow dung were characterized and then estimation of biogas production and methane content of each treatment, T1 (cow dung alone), T2 (1:1), T3 (2:1), T4 (3:1), T5 (JS alone), T6 and T7 (with digester effluent) was performed using indirect (water displacement) and absorption of CO2 by 10% NaOH methods, respectively. Statistically significance difference (at 0.05 levels) on production of biogas among treatments was observed. It was found that T5 (JS alone) was highest in the amount of biogas production but lowest in its quality (i.e., methane content) and T3 (2:1 ratio of cow dung to JS) was the second highest in the amount of production, but the highest in quality. Thus, T3 produced the optimum methane gas among treatments. Moreover, JS and its combinations with cow dung produced higher volume of biogas and contained more macro-nutrient in the slurry for plants than cow dung alone. Thus, JS appears to be a good material for biogas and bio-slurry production.