E36 high-strength steel and 304 austanitic stainless steel are welded by fiber laser. Microstructure is investigated by means of optical microscopy (OM), energy disperse spectroscopy (EDS), scanning electron microscopy (SEM) and X-ray Diffraction (XRD). Micro hardness of weld is tested by micro hardness tester. The results show that the weld joint of dissimilar steel with good quality could be obtained by fiber laser. In the position of 304 fusion zone, d-ferrites are gradually decreased with the increase of welding speed. The microstructures of welding metal close to 304 stainless steel is planar crystal while that close to E36 and welding center are columnar crystal. With the increase of welding speed, the quantity of cellular crystal of welding metal close to 304 stainless steel is increasing and so does cellular crystal of welding center, but there has no significant change in weld microstructure near the E36. Weld zone is mainly consisted by martensites and a small amount of carbides. Microstructural evolution caused by welding speed is not obvious. Composition of welding line zone is well-distributed. However, there is composition gradient on both sides of the fusion zone,which is influenced significantly by welding speed. Lath-shaped martensites ferrites and pearlites are produced on the heat affected zone of E36. The highest hardness value appears on welding center .The hardness of weld zone and E36 heat affected zone is increasing with the increase of welding speed.