The gene smu.776 encodes a possible S-adenosylmethionine-dependent methyltransferase of 385 residues in Streptococcus mutans, a primary pathogen for human dental caries. The DNA fragment of smu.776 was cloned into pET28a and expressed in good amount from the E. coli strain BL21 (DE3). Smu.776 protein was purified to homogeneity in a two-step procedure of Ni²⁺ chelating and size exclusion chromatography. Crystals were obtained by hanging-drop vapor diffusion method and diffracted to 2.0 Å resolution. The crystal belongs to orthorhombic space group C2 with cell dimension of a=168.47 Å, b= 50.66 Å, c=53.96 Å, β=104.22°. The asymmetric unit is expected to contain one molecule with solvent content of 51.3%.