Ultraflattened dispersion of optical fibers is urgently needed for ultrawideband and ultralong-haul transmission in dense wavelength division multiplexing (DWDM) systems. A systematic solution is finely developed to adjust and control the ultraflattened dispersion curves in high accuracy. The validity is verified by designing two photonic crystal fibers (PCFs) with ultraflattened chromatic dispersion across the whole telecom band (1260 to 1625 nm). Numerical results confirm that line PCF with near-zero, all-positive, and ultraflatten dispersion ranging 0.32  ±  0.17  ps  /    (  nm  ·  km  )   can support 1600-km line transmission in DWDM, whereas the dispersion-compensating PCF with low negative and ultraflattened dispersion ranging −15.57  ±  0.37  ps  /    (  nm  ·  km  )   has a high dispersion compensation efficiency of up to 33 times for line PCF. An improved PCF structure is proposed by introducing three extra small air-holes between the standard lattices of odd rings. Moreover, other fiber parameters such as nonlinear coefficient (<1.1  w^−  1  ·  km^−  1), numerical aperture (>0.189), and confinement loss (<0.09  dB  /  km for line PCF) implied that both PCFs presented can support long-haul linear scheme in the whole telecom band.