Abstract: Effect of methanol on inorganic nitrogen conversion enhancement and bacterial diversity in sediment from shrimp pond was investigated. Sediment from shrimp pond in Pathum Thani, Thailand, was packed in 40 cm height cylinder shape chamber, filled with 1.4 L of 20 g L-1 seawater and incubated under dark and aerated conditions for 48 days. The experimental units consisted of (1) control chambers without methanol addition, (2) treatment-1 chambers supplemented with methanol at C:N ratio of 2:1 and (3) treatment-2 chambers with methanol at C:N ratio of 4:1. It was found that, methanol addition could stimulate ammonification process so ammonia was released from sediment at higher rate than control. Thereafter, concentration of nitrogenous wastes i.e., ammonia, nitrite and nitrate were decreased due to sequential nitrification-denitrification processes. However, methanol addition seemed to inhibit nitrification process since treatment chambers took longer time to reduce ammonia and nitrite. Nitrogen conversion pattern were similar in both treatments with C:N ratio of 2:1 and 4:1. Bacterial community in sediment, as analyzed by PCR-DGGE of 16S rRNA gene fragments, showed that DNA profile at the beginning and the final day of the experiment was clearly changed, while bacterial profiles of treatment 1 and 2 at day 48 was almost similar. On the other hand, addition of methanol at the high C:N ratio reduced the bacterial diversity in which the Shanon-Weaver index was found lowest. DNA sequencing results indicated that denitrifying bacteria such as Vibrio sp., Psuedomonas sp., Planococcus sp., Streptomyces sp. and Thioalcalovibrio sp. were stimulated after methanol addition.