Purpose: To study dynamic characters and functions of bio-molecules at the single molecular level.Methods: Total internal reflection fluorescence microscopy was refined to directly visualize single fluorescent dye molecules in solution. Nanometer-scale motions of single molecules were detected by optical trapping nanometry.Results: We succeeded in visualizing single fluorescent dye molecules in aqueous solution at a full video rate. This method enabled us to visualize motion of single motor protein molecules and the individual ATPase reactions (Funatsu, et al., 1995 Nature, 374,555; Vale et al., 1996Nature, 380:451). Conformational diversity of individual motor protein molecules was imaged by a fluorescence resonance energy transfer.Conclusions: The technique for visualizing single molecules and its combination with nano-manipulation developed here could be widely used to measure motions and chemical reactions of DNA motors, ion pumps, ATP synthase, etc. directly at the single molecular level.