Abstract: This paper states the condition about gas pipe and the flaws of gas pipe, and based on this it describes the development and presents problems about In-line Inspection of Natural Gas Pipeline. Reviewing the characteristics and difficulties, this paper analyzes MFL, RFEC and EMAT, besides it introduces two new NDT Technologies: MMMT and LUT. At last, by comparing the characteristics and detectability of all NDT Technologies, then this paper also points the development of NDT Technology for In-line Inspection of Natural Gas Pipeline.