Detection of 21 cm emission/absorption signatures of neutral hydrogen at z=6-50 gainst cosmic microwave background will provide a unique tool for study of ormation of first stars/QSOs in the universe, for understanding of reionization istories and for mapping of 3D matter distribution at high redshifts. We are building huge array of 10160 2-meter log periodic antennas for measurement of the 21 cm mission from reionization epoch. The array, 21CMA, consisting of 80 pods with a aseline of 3 km, allows us to reach an angular resolution of 4 arcmin and a ensitivity of about 1 mK per day. 1/4 of the array has been set up in Xinjiang, west China and starts to collect the weak 21 cm background signals.