Abstract: The ideality factor of GaN LED on Si substrate was studied.By studying the relation between I-V characteristics of different samples and their crystal properties,which were characterized by the full-width at half-maximum（FWHM）of the Δω-rocking curve for GaN（102） double crystal X-ray diffraction peak,it is found that there is direct relation between the ideality factor and the crystal property: the ideality factors for the three samples are 6.6,4.5,3.0,respectively,which are corresponding to the（102） DCXRD FWHM of each sample : 707,530,401,respectively.The anomalously high ideality factors（n≥2.0） are contributed to the high defect density,which makes tunneling current take place more easily.