Abstract: A method edits a PLC program in a computer with a pointing device such as a mouse. When a drag-and-drop operation is detected, a dragged source is identified. When the dragged source is from libraries, the dragged source is optionally converted into an IEC61131-syntax program portion according to the type of the libraries, namely, function or function block. When the dragged source is a POU (Programming Organization Unit), the dragged source is optionally converted into an IEC61131-syntax program portion according to the source POU type, the currently-edited POU type and criterion of forbidding recursion call. When the drag and drop operation is not feasible, the shape of mouse cursor is changed to remind user. The method of the present invention can advantageously convert items in libraries and POU into IEC61131-syntax program portion to facilitate the PLC programming task.