Initial sizing is one of the important tasks in conceptual design of civil jets. The aim of this paper is to develop an initial sizing tool for rapid conceptual design of civil jets. Through literature review and theoretical analysis, the initial sizing method of civil jets is presented. The method needs only the simple inputs, including payload, range, flight performance requirements and a few pre-estimated parameters. The wing loading, thrust-toweight ratio and weight properties of the jets can be estimated. Based on the method, a tool with friendly graphical user interface was developed by use of MATLAB programming. The validation cases show that the results determined by the tool are fairly consistent with the reference jets.