Thanks Dear Michael, but, Other researchers agree with Michael?

The reason mentioned by Michael is one certainly. But, free radicals and oxidative stress is problem faced by genetic material in almost every cell. I think the bigger reason is problem in fidelity of polymerase. In fact, as reported by song in 2005

A nuclear gene, called DNA polymerase gamma (POLG), encodes the DNA polymerase responsible for replicating the mitochondrial genome. The POLG protein consists of two domains: a catalytic domain that exhibits polymerase activity, and an exonuclease domain that is involved in the recognition and removal of DNA base-pair mismatches that occur during DNA replication. A recent study suggests that mitochondria may have a nucleotide imbalance that leads to decreased POLG fidelity and higher mitochondrial DNA mutation rates.

You should in fact read this research article

"DNA precursor asymmetries in mammalian tissue mitochondria and possible contribution to mutagenesis through reduced replication fidelity." Proceedings of the National Academy of Sciences by Song S. et. al.

Thanks Dear Satya, Yes, it can be a reason of susceptibility to mutation, but are there Other reasons ? Can anybody please guide me?

Very True Michael. There is a combination of contributing factor. I think Reyhaneh is looking for something specific. It will be nice if you present your possible reason. May be me and Michael can be of more help then....

Dear Satya and Michael

Thanks you for suggestions and helps, sorry but the reasons don't satisfy me! I Know about DNA polymerase gamma, Size of mtDNA, dNTP pool, reactive intermediary products, haploid inheritance and mtDNA lacks intron , however, I would you like to know whether there are further reasons or not. But thanks again for everything.

The POLG low fidelity and higher exposure rate to Oxidative agent made mtDNA pron to mutations. Despite of them, mtDNA is the most informative in phylogentic studies and sequencing based mutation rate for mtDNA in control region is .0043 per generation.