A height adjuster of a seat belt for a vehicle may include a rail bracket configured to be fixedly mounted on a vehicle body so that rail grooves are formed to be continuously connected and the rail grooves are vertically disposed on the rail bracket, a slider configured to be coupled to the rail bracket and moving along a length direction of the rail bracket and having left and right surfaces provided with guide grooves so as to be fitted in edge portions of the rail bracket which are provided with the rail grooves, and a locking mechanism configured to be coupled to the slider so as to elastically slidably move and restrict a movement of the slider at a time of contacting the rail groove and release the restriction of the movement of the slider at a time of being separated from the rail groove.