Non-small cell lung cancer (NSCLC) patients, with sensitive epidermal growth factor receptor (EGFR) mutations react well to tyrosine kinase inhibitors (TKIs). However, the efficacy of TKIs on patients with the same mutant types differs dramatically. It is implied that the different quantities of mutant alleles could be one of the reasons underlying. Patients with high abundance of EGFR mutation might benefit more from TKIs. There are no universal standards for the definition of EGFR mutant abundance. Abundance could be semi-quantified according to the different sensitivities of detection methods, quantified with quantifying detection techniques such as digital PCR or next generation sequencing, or quantified based on the expression of mutant proteins. The different abundances of primary and metastatic diseases could reflect the heterogeneity of the tumors. The pre-treatment level or the dynamic change of EGFR mutant abundance could help observe the course of the diseases and predict the efficacy of TKIs. TKIs resistance could be detected by change of abundance prior to image manifestations. Besides, the abundance of T790M could also predict drug efficacy and resistance of the first and third generation TKIs. Thus the detection of EGFR mutant abundance has important clinical significance. The standardization and correction of abundance needs more exploration.