Considering day to day increase in the cost of the gasoline for Iranian customers; the interested for using dual fuel automobiles are increasing. It is common using steel CNG vessels in Iranian dual fuel cars. Nowadays; there are millions of CNG vessels with more than 10 years of commissioning time in Iran. Recently, periodic inspection regulations are issued by the Iranian Standard bureau which is mandatory for taxi services. In this regulation, visual inspections are considered and it is recommended using ultrasonic inspections and/or hydraulic tests for critical vessels. Because of the importance of the topic for the safety of the citizens, this paper dedicated to investigate the statistics of the CNG vessels and their most common defects in country. In addition, the periodic inspection procedure of the Iranian regulations is compared with international counterparts. According to the discussion, inspection measures which considered by the Iranian standard bureau are enough and useful for standard CNG vessels; however, more than 25% of all CNG vessels in Iran are manufactured in nonstandard manners and they need more inspection measures. It seems that reducing of the inspection period from 36 months to 24 months will be of value.