Abstract: There are many methods to improveg array directivity. However, different methods developed before are difficult to resolve the following paradox: To narrow the main lobe will surely increase side lobes. On the other hand, to reduce side lobes will widen the main lobe, and hence, reduces the directional index. This paper develops a time-space synthesis process by forming an improving factor. The method could be used both to narrow main lobe and to reduce side lobes, and therefore to increase directional index. Using this method, two cases （16 elements and 32 elements） were calculated. The results show that the side lobes are reduced and the main lobes are narrowed at the same time. Thus the directional index is increased from 1.5 dB to 2.5dB. This paper shows) that thel method is also useful for sweeping of main lobe in wide frequency band.