Slippage, crack nucleation, propagation are analyzed for zircaloy4 during monotonic and cyclic deformation by means of insitu SEM observation. The results show that in the course of tension testing the slippage becomes heavier, and changes from single slip to multiple slips as the applied stress increases. Twist plays an important role in maintaining homogeneous deformation in the final deformation stage. The specimen fracture occurs as soon as the cracks reach the specimen surface. During cycling, a number of cracks nucleate, propagate and link in the vicinity of the grain boundaries. TEM observation shows that a typical dislocation configuration is parallel dislocation lines in transverse section and embryonic dislocation cells in longitudinal section.