An in situ photochemical fluorescence probe method for the determination of nucleic acids with vitamin k3(VK3) as the photochemical fluorescence probe was developed for the first time. It was based on the conversion of VK3into an intensively fluorescent product on irradiating with UV radiation. The photochemical reaction is decelerated by nucleic acids. The determination can be carried out by measuring the fluorescence intensity at a fixed time. The calibration graph was linear in the range of 0– 1.5 μg/ml for CT DNA and 0–2.0μg/ml for yeast RNA, the limit of detection was 10 ng/ml for CT DNA and 26 ng/ml for yeast RNA. The kinetic behaviour of the photochemical reaction and the effects of some experimental conditions were investigated and discussed in detail. CT DNA could be determined in the presence of 40%(w/w) yeast RNA and yeast RNA was determined when the content of CT DNA in synthetic samples was below 6%(w/w).