This paper presents a general method of programming ladder diagrams by the step transition control diagram. It solves the programming problem of Programmable Gontrollers. ingeniously. The sample programs presented in this paper have been all verified on a PLC-2/30 programmable controller. Results show the presented method is correct and reliable. The method has some striking features. It is simple, clear, rigorous, regular and applicable. It is really worth its weight in gold during programming ladder diagrams for PC users.