Objective: To study the relationship between genetic diversity within region of surface protein P1 of Streptococcus mutans (serotype c) isolates and their binding abilities to salivary acquired pellicle. Materials and Methods: The clinical isolates of S. mutans (serotype c) included two groups with different binding abilities (cpm>2000, cpm<1000), which were derived from previous studies in our experimental lab. The bacterial DNA was extracted, and spaP-a (473-1571bp) was amplified by PCR. Genetic diversity of spaP-a was assessed by restriction fragment-length polymorphism (RFLP) with restriction endonuclease Hae III. Results: Four different patterns of spaP-a PCR-RFLP among strains were revealed when digested with Hae III. The distribution of four genotypes of strains with different binding abilities were different (p<0.05). Conclusion: The genetic diversity exists within spaP-a (473-1571bp) of the clinical isolates of S. mutans (serotype c). The diversity of strains with higher binding abilities was more apparent than those with lower ones.