we demonstrate a novel integrated optical fiber interferometer for in-fiber optofluidic detection. it is composed of a specially designed hollow optical fiber with a micro-channel and two cores. one core on the inner surface of the micro-channel is served as sensing arm and the other core in the annular cladding is served as reference arm. fusion-and-tapering method is employed to couple light from a single mode fiber to the hollow optical fiber in this device. sampling is realized by side opening a microhole on the surface of the hollow optical fiber. under differential pressure between the end of the hollow fiber and the microhole, the liquids can form steady microflows in the micro-channel. simultaneously, the interference spectrum of the interferometer device shifts with the variation of the concentration of the microfluid in the channel. the optofluidic in-fiber interferometer has a sensitivity of refractive index around 2508 nm/riu for nacl. for medicine concentration detection, its sensitivity is 0.076 nm/mmoll−1for ascorbic acid. significantly, this work presents a compact microfluidic in-fiber interferometer with a micro-channel which can be integrated with chip devices without spatial optical coupling and without complex manufacturing procedure of the waveguide on the chips. © 2017 optical society of america.