A study was carried out by using both syringes and flasks for measuring in vitro biogas production and quality with the basal substrates of pig manure (15%DM). In the experiment of syringes the factorial design was used. The first factor was the plant material including rice straw, water hyacinth and Para grass (Brachiaria mutica). The second factor was replacement levels of plants to pig manure including 10, 20, 30, 40 and 50% (DM basis) with three replications. In the experiment of flasks, corresponding to each treatment of the experiment of syringe system, one 5liter-flask was used. The inoculum was extracted from pig sludge of flasks which were incubated at 40^oC at day 80. Incubation time for the study lasted 60 days. The results showed that the potential biogas production of rice straw and water hyacinth were better than Para grass. The replacement of plant level at 50% to pig manure as the main substrate gave the highest biogas production in both syringe and flask systems. Methane concentration reached 60% of biogas for the syringe was earlier than the flasks (day 14 vs. day 28). The syringes could be potentially used to study on in vitro biogas production. Studies on replacement of pig manure by these plants more than 50% (DM basis) in biogas production should be considered.