The present invention provides a steel piston having cooling channel without flash, including an upper piston part having a circle shape outer wall and an inner wall formed inward from the outer wall with a desired distance, and a lower piston part having a circle shape outer wall and an inner wall formed inward from the outer wall with a desired distance on the place corresponding each other when they are coupled, in which one or more concave grooves are formed on the outer wall of the upper piston part, and two concave grooves are formed on the inner wall of the upper piston part, and the inner circumference surface of the upper end of the outer wall of the lower piston part is carved as much as a desired depth, and therefore, a stepped portion is formed between the inner and the outer circumference surfaces, and the inner and outer circumference surfaces of the upper end of the inner wall of the lower piston part are carved as much as a desired depth, and a protrusion is formed at the middle of the upper end of the inner wall, and therefore, the stepped portions are formed between the inner and outer circumference surfaces, and therefore, having the effect that improve the cooling efficiency of the piston by preventing from accumulating of carbides by limiting forming flash inside oil gallery during friction welding.