Abstract: As a kind of surface modification methods, femtosecond laser can create micro-structures on metal surface by reducing materials. Femtosecond laser surface modification method has the characteristics of high processing accuracy and low thermal impact. Compared with large wavelength laser beam, the melted area caused by femtosecond laser is smaller, and the impact on the original substrate surface is weaker. The modified surface has the potential to enhance heat transfer, while investigation on the mechanism of heat enhancement is insufficient. Visual data can show nucleation, growing, sliding and lift-up of bubble on heating surface which is related to the micro-structure. However, many visual experiments were carried in pool boiling conditions. Research on flow boiling visualization was relatively less. Using modified surface manufactured by femtosecond laser in rectangular channel, the visual experiment in 2 MPa environment was carried. Because of the influence by pressure and flow, the bubble size achieved 0.05 mm and high-speed camera was applied. According to the length calibration, the area of image was 114.7 mm² and the distance between 1 pixel achieved 0.01265 mm. Owing to camera with 4 000 fps, the motion of bubble was recorded. In flow conditions, bubble slides along heating surface after leaving nucleation point. Visual data show that after leaving nucleation site bubble slides in groups on ordinary surface and modified surface. Due to the more micro-structures on modified surface, the shape of bubble is changed continuously when contacting with micro-structures on sliding trajectory. Based on images, the bubble nucleation site density on ordinary surface and modified surface were calculated. The influences of inlet mass flow and temperature on nucleation site density are not significant. The intend of nucleation site density on ordinary and modified surface with wall superheat complies with exponential trend. As nucleation cavity, the micro-structures on modified surface causes more nucleation site density than ordinary surface with same wall superheat. With the improvement of wall superheat, the difference of nucleation site density between modified surface and ordinary surface grows. Besides, experimental data and calculation by reference models were compared. As a kind of hydrophilic surface with plenty of nucleation cavities, using contact angle in some models might receive less nucleation site density, which is contrary to the experimental results. This conclusion shows that wettability is not a virtual factor for nucleation site density on femtosecond laser modified surface which is extremely rough. As a consequence, the model of nucleation site density adapted to modified surface is founded. Compared with experimental data, 80% of conditions was within 30% erro margin. The new model reflects that modified surfaces with the same wall superheat conditions have more nucleation points per unit area compared to conventional surfaces by using surface hole density.