Abstract: According to the principle of evanescent wave and the distribution of optical light from the side polished area of side polished fiber, an all fiber online optical power monitor was made by packaging a chip of optoelectronic detector on the side-polished fiber with U shape and suitable depth to detect the leaked light. Using the wheel side-polished fiber machine, side-polished fibers with the special shape and depth of polished area for monitoring optical power in the fiber was manufactured. The measured results show that the optical power monitor features good optical responsivity and its maximum opto-electric transition efficiency reaches 200mA/W. Its wavelength dependent loss is 0.3dB（1520nm～1620nm）, polarization dependent loss is 0.07dB. The all fiber optical power monitor has no invasion to the fiber core, no insertion elements and is easy to be connected with the optical system directly.