Effect of the laser chirp on the laser filamentation is investigated. It is found that the chirp plays a crucial role in the formation of filamentation and its evolution. An initial negative chirp is favorable for long-distance propagation of filaments in air. As the chirp increases, the distance between the compressor and the position of the onset of filaments becomes larger. We also investigate the influence of initial chirp on the focusing laser pulses by a lens. It is found that there are the same tendency of filaments formed by lens and those in free propagation.