Objective    To detect the microsatellite instability (MSI) of mitochondrial DNA (mtDNA) in lung squamous carcinoma and to investigate its significance in carcinogenesis.     Methods    Carcinoma tissues were obtained from 15 cases of lung squamous carcinoma patients and mtDNA was extracted by one-step method. D-loop fragments were amplified by PCR. mtMSI were determined by DNA sequencing.     Results    The mtMSI as detected in all of 15 lung squamous carcinoma patients. Thirty mtMSIs were found in 3 different nucleotide regions.     Conclusion    These results indicate that the frequency of mtMSI in lung squamous carcinoma patients is very high and mtMSI in D-Loop might play an important role in lung carcinogenesis.