The common practice for determination of orientational angle of interfacial molecular groups with the interfacial sum frequency generation vibrational spectroscopy (SFG-VS) is to measure the intensity ratio between two SFG intensities at a certain vibrational frequency with different polarization directions. Because sometimes the SFG at one polarization direction is too weak to be measured accurately, this ratio usually has big uncertainty, and consequently it is impossible to obtain accurate orientation parameters. Thus the corresponding orientation angle was not accurately calculated. Through analyzing the basic relationship between the orientation angle and SFG intensity, we found that the null angle method would be suitable for such cases. The null angle measurement is simple, precise and easy to realize, therefore can greatly improve the accuracy of orientation angle. Using this method, we determined the orientation angle of the methyl group of methanol orients about 31.8 ± 2.4° from the surface normal at the air/methanol interface, much more accurate than the previously reported value of ＜42°. The null angle method provides a reliable experimental analytical tool for studying the orientation of the chemical group or chemical bond at interfaces.