Abstract: Factors to be considered in materials selection for high strength bolts are discussed in view of their service conditions. Axial and inclined tensile, impact, tensile fatigue and delayed fracture tests were made on low carbon martensite steels and tempered medium carbon steels. The experimental results have shown that the strength of lowcarbon marten sites is about 1/3 higher than that of 600℃ tempered medium carbon steels, while maintaining comparatively higher ductility and toughness; the load carrying capacity of bolts made from low carbon martensite steels is 45-70% higher as compared with that made of 600℃ tempered medium carbon steels, but the former shows no higher sensitivity to notches and inclination over the latter; fatigue strength of bolts made from both steels is basically the same; low carbon martensite steels have lower sensitivity to delayed fracture than steel 40Cr at the same strength level, and are insensitive to delayed fracture in brine and water, Therfore to choose low carbon martensite steels for making high strength bolts is advantageous not only for their superior comprehensive mechanical properties, but also for their much better forming characteristic. Tests were made on the influence of rolling thread before or after heat-treatment upon the fatigue strength of bolts. Results have shown, rolling screws after heat-treatment can effectivelly increase their fatigue strength. According to these experimental results, the auther also presents a discussion on composition design for low cardon martensite bolt steels.