This research aims to contribute the characterization of acetolactate synthase (Ec 4.1.3.18; ALS) and the resistance mechanism by sequence analysis of ALS gene of the sulfonylurea-resistant and -susceptible Monochoria vaginalis. The ALS gene was obtained from susceptible (S) and resistant (R) M. vaginalis to sulfonylurea herbicides (SUs). The 815 bp the fragment and the genomic DNA sequence coding for acetolactate synthase (ALS) of S and R biotypes of M. vaginalis were cloned and sequenced. Nineteen clones were divided greatly into 4 groups as result of sequencing. The first group was not difference to S type, the second group was amino acid of P197S which found point mutations causing substitution of serine for proline at amino acid 197, the third group was observed greatly other part of 6 places than group 1, and the fourth group appeared the intergrade of group 1 and 3. Therefore, it could be assumed what ALS gene of various types can be one plant. The peptide of the 13 amino acid Domain A region for ALS genes from R biotype of M. vaginalis differed from that of the S biotype by one base substitution at proline codon of Domain A. It could also be confirmed that point mutation of serine for proline at amino acid 197.