With the help of isothermal compression tests and high-speed camera, the critical fracture reduction was determined and the fracture mode was verified in hot compression of Ti40 alloy in the deformation temperature range of 850~1100 oC, strain rate range of 0.01~10 s-1 and at the reduction of 70%. And the fracture mechanisms were investigated. It is found that the high-speed photography can shoot accurately the crack initiation and propagation, and observe the different fracture modes such as 45o shear cracking, V-type and longitudinal cracking to high-speed photography. Optical microscope and SEM were employed to observe the fracture morphologies. The results show that at high strain rate, it is mixed type brittle fracture in low-temperature region, whereas intergranular cracking with small dimple in middle-temperature region, and ductile fracture with a large number of dimple in high-temperature region. Moreover, the effect of deformation temperature and strain rate on fracture patterns and crack growth rate is significant. Fracture modes change from 45o shear cracking to V-type and longitudinal cracking and critical fracture reduction increases with the increase of temperature, while crack growth rate increases and critical fracture reduction decreases with the increasing of strain rate. It is suggested that high-speed photography is effective for research on the fracture initiation and crack propagation of Ti40 titanium alloy. Meanwhile, the exploration to the fracture mechanisms in hot compression of Ti40 alloy is of great significance for the study of deformation mechanism, optimization of processing parameters and process-microstructure-property in hot working of Ti40 alloy