The 27 Al+ ion optical clock is one of the most attractive optical clocks due to its own advantages such as low black-body radiation shift at room temperature and insensitivity to the magnetic drift. However, it cannot be laser-cooled directly in the absence of 167nm laser to date. This problem can be solved by sympathetic cooling. In this work, a linear Paul trap is used to trap both 40 Ca+ and 27 Al+ ions simultaneously, and a single Doppler-cooled 40 Ca+ ion is employed to sympathetically cool a single 27 Al+ ion. Thus a'bright-dark'two-ion crystal has been successfully synthesized. The temperature of the crystal has been estimated to be about 7 mK by measuring the ratio of carrier and sideband spectral intensities. Finally, the dark ion is proved to be an 27 Al+ ion by precise measuring of the ion crystal's secular motion frequency, which means that it is a great step for our 27 Al+ quantum logic clock.