We studied collagen reversible denaturation by a weak near-infrared laser light irradiation for vascular softening. Balloon angioplasty induced about 60% restenosis rate for superficial femoral artery at 6 months after the balloon dilatation originated from mechanical injuries in vascular wall. We anticipated that mechanical injuries can be prevented by softening of the collagen fibers in vascular wall. Collagen softening is resulted from dissociation of a few hydrogen bonds in collagen molecules in general. We estimated that hydrogen bonds can be dissociated by near-infrared photon energy with less heat production because hydrogen bond energy is about 30 kJ/mol corresponding to about 4 μm in wavelength. We studied collagen denaturation by a weak near-infrared laser light irradiation from mechanical and optical characteristic changes. A collagen film derived from bovine dermis was irradiated with 980 nm wavelength semiconductor laser under the condition that temperature rise was kept under 4.1°C. To evaluate collagen denaturation from mechanical characteristic change, the elastic modulus was analyzed from the stress-strain curve of the irradiated collagen film. Stress of the collagen film was measured with a load cell when 0.25%/s strain was loaded with a rack-andpinion automatic stage. Infrared absorption spectrum peak in 3300 cm-1 band of the collagen film, which is originated from hydrogen bonds, was measured to evaluate collagen denaturation from optical characteristic change. We found that the collagen film was 12% softened by a particular laser light irradiance.