Abstract: the bending losses rise greatly of the longer wavelengths, inspite of the dependence of the wavelength is extremely strongly on the oscillatory because of the overlap with the reflected lightin the coating boundary or cladding as well as from the outer of the coating surface. The growing of bending losses at longer frequently of the wavelengths defines as the suitable range of the wavelength for the single-mode fiber. An experimental investigation was made of the bending losses and the calibration losses in a single-mode fiber using two types of different light sources that were the pure red laser diode with range of 635 nm as wavelength and the infrared light-emitting diode operate between 940-950 nm. The measurements were carried out at the Communication and Postal Service, Al-Hillah-Babylon-Iraq. the bending losses, the calibration losses and the curvature critical radius were considered in this study. the results were contrasted with the theoretical data in order to investigate out that the both finding were very close.