We report measurement of heating rates of 40 Ca+ ions confined in our home-made microscopic surface-electrode trap by a Doppler recooling method.The ions are trapped with approximately 800 μm above the surface,and are subjected to heating due to various noises in the trap.There are 3 5 ions involved to measure the heating rates precisely and efficiently.We show the heating rates in variance with the number and the position of the ions as well as the radio-frequency power,which are helpful for understanding the trap imperfection.