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High resistance GaN thin film was grown on sapphire(0001) substrates using metal-organic chemical vapor deposition(MOCVD).Effect of the GaN nucleation layer growth parameters,including reactor pressure,species of carrier gas and growth time,on the electrical characteristics of the following grown GaN buffer was investigated.It is found that GaN films epitaxially grown on the GaN nucleation layers deposited at a relatively lower pressure tend to have a high resistance.High resistance GaN buffer layer can also be prepared by extending growth time of the nucleation layer(i.e.increasing the thickness of nucleation layer) or by using N2 instead of H2 as carrier gas during the growth of nucleation layer.GaN layers with a sheet resistance as high as 2.49×1011 Ω/□ was obtained.These layers were used as templates for the preparation of epi-wafers with AlGaN/AlN/GaN hetero structures,which were used to fabricate high electron mobility transistors(HEMTs).The highest mobility of these samples reaches to 1 230 cm2/(V·s).