Objective        To evaluate the impact of photodynamic therapy on the viability and morphology of Streptococcus mutans in a planktonic culture.       Methods        Inhibitory action of toluidine blue O-mediated photodynamic therapy on Streptococcus mutans were analyzed by colony-counting method under different light intensity in vitro. Cell morphologic changes of Streptococcus mutans was observed under scanning electron microscope. Samples were prepared and divided into control group

photosensitizer control group

light control group and photodynamic therapy group.       Results        Photodynamic therapy resulted in a significant reduction in the bacterial growth in a irradiation time- and photosensitizer concentration-dependent manner. When the photosensitizer concentration was 100 μg/ml

irradiation time of 5

15 and 25 min had colony log value of Streptococcus mutans for 7.40±0.13

6.86±0.14

and 6.44±0.28 respectively

which were significant lower than control group (8.30±0.04

P0.05). Interaction was found between photosensitizer concentration and the light dosage（F=19.47

P0.01）. Scanning electron microscopy displayed that the bacterial cell surfaces of photodynamic therapy group became rough

with blister shaped bulge.       Conclusion        Photodynamic therapy with toluidine blue O as photosensitizer exerts efficient inhibitory effect on the growth and morphology of Streptococcus mutans.