Objective(s): As mitochondrial oxidative stress is probably entailed in ATP production, a candidate modifier factor for the long QT syndrome (LQTS) could be mitochondrial DNA (mtDNA). It has been notified that ion channels' activities in cardiomyocytes are sensitive to the ATP level. Materials and Methods: The sample of the research was an Iranian family with LQTS for mutations by PCR-SSCP and DNA sequencing. The study searched about 40% of the entire mitochondrial genome in the family. Results: Four novel mutations that lead to an amino acid substitution and two mutations in mitochondrial tRNA have been informed in this study. A Statistically significant correlation (r = 0.737) between QTc (ms) and the age of LQTS patients has been reported. Conclusion: The research data show that these mitochondrial mutations, in a family with LQTS, might be the responsible mitochondrial that defect and increase the gravity of LQTS.