The chirped Mo/Si multilayer mirror in the extreme ultraviolet region is designed to obtain sub-femtosecond pulses from high-order harmonics. Numerical simulations of temporal profile of the pulses are made for superposition of incident high-order harmonics and that of reflected ones by the chirped multilayer mirror. The normal incidence reflectivity and chirp in the wavelength range of 12.5-16.5 nm are 6.7%+-0.5% and -3617+-171 as2, respectively. Simulation results indicate that the designed chirped multilayer mirror can be used for producing 104-as pulses.