Abstract: Mode-field diameter (MFD) is a characteristic parameter of fundamental mode in fiber-optics waveguide. Power density in single-mode fiber is inversely proportional to the MFD. As the single-mode output power of fiber laser increasing, the power density in the fiber increases intensely. Much higher power density can cause optical damage and thermal damage of fiber waveguide. Based on the approximate mode-field distribution model, a character was found that the minimum MFDs in tapered fibers with different original fiber parameters corresponded to an invariable fiber normalized frequency. The relations between the core diameters and MFDs were simulated by the finite difference beam propagation method (FD-BPM) in different wavelengths and in different core numerical apertures (NA). The results demonstrated the supposition presented above and indicated that the minimum MFDs corresponded to the normalized frequency 1.8 nearly, without direct relations with the wavelength and fiber para-meters. The results provide theoretical reference for the quick determination of the point of maximum power density in fiber, especially in high power case, as well as add new content to the fiber optics theories.