Abstract: According to the theory of ABC system, theoretical spectra of ABX system were calculated by varying|J|< 16Hz and the positions and intensities of each specific transitions were analyzed The general rules of the position of each transition in ABX system with various values of coupling constants and their signs were found out Then, a simple procedure was designed by which relative signs of couping constants can be obtained without calculating precisely through intensities of each peak, as follows:
1. To distinguish the four peaks of each A, B, X nuclei in ABX system and to give their marks in order, respectively. (For example, from left to right, a₁~a₄,b₁~b₄,and x₁~x₄, neglecting the mixing transitions.)
2. To get a related diagram discribing the relationship between each peak using double resonance methods.
3. Through inter-exchanging of the marks of the same kind of peaks, finally you can always let a₁, b₁, x₁ to be at the bottom and a₄, b₄, x₄ on the top as a usual diagram of energy level.
4. If you don't need to inter-exchange any marks of peaks, the signs of three coupling constants are the same; if you need to do, then, the sign of coupling constant between relative nuclei due to inter-exchanging must be opposite sign to the other two.

Key words: NMR, ABX system, Transition position, Coupling cinstant, Relative signs, Relationship