Mg contents of Zn1-xMgxO film grown on A-sapphire substrates by molecular beam epitaxy were measured by inductively coupled plasma (ICP) method. Through theoretical analysis, an expression for the difference of Mg content in Zn1-xMgxO film calculated by simple and quadratic inspection formula was given. By comparing the measured results of the ICP with electron probe microanalysis (EPMA), the consistency of ICP with simple inspection formula and EPMA was deduced when Mg content in the samples is less than 0.5, thus the correctness of the data measured by ICP was validated.