A head up display for vehicles includes: at least one light source; at least one light guide set in front of the light source; an image panel set in front of the light guide; and a surface with the feature of at least partial specular reflection. Wherein the light emitted from the light source illuminates the image panel via the light guide to project an image on the image panel onto the surface and then be reflected to an observer, so as to form a virtual image observed by the observer. The light guide of the present invention can reduce the light loss of the head up display for vehicles and make the illumination light on the image panel more homogeneous and possessing high directionality, and so as to improve the brightness and quality of the virtual image formed by the head up display for vehicles.