Optical fiber sensing technology is one of the very promising techniques in sensing fields. A high sensitivity high temperature sensor based on inline optical fiber Mach-Zehnder(M-Z) interferometer by using standard single mode fiber with two waist-enlarged bitapers is proposed in the present paper. The waist-enlarged bitapers are considered as couplers, the distance between the two bitapers is the sensing arm. The light in the lead-in fiber core couples into the sensing arms’ fiber core and cladding by the first bitaper, and then propagate in them. The phase difference between core mode and cladding mode is produced when the light reaches the second bitaper. Then the second bitaper couples the light into the lead-out single-mode fiber to get the interference spectrum. The sensors with different length were fabricated. The relationship between the sensor length and interference period, and the temperature response of the sensor were studied by experiments. The results show that the 35 mm long sensor has a high sensitivity of 0.115 nm·℃-1 in the range of 30～400 ℃. The transmission spectrum of the sensor was also analyzed by the fast Fourier transform. It shows that only LP01 mode and LP08 mode propagate in the sensor. The sensor has advantages of small size, high precision, and immunity to electromagnetic inteference. In addition, it is of easy fabrication, high signal-to-noise ratio, light weight, and high sensitivity, and could be operated under high temperature. This kind of sensor is a good candidate for high temperature measurement of hot gas, oil and gas well logging and other areas.