We have redefined the $alpha$ -parameter of the extended transmitters in the presence of self-phase modulation (SPM) induced chirp. The $alpha$ -parameter is effectively lowered to about 1, when the SPM effect with the launching power of 12.5dBm (maximum launching power to avoid stimulated Brillouin scattering(SBS)) is included, compared to the $alpha$ -parameter of the transmitter algone. The lowered $alpha$ -parameter has improved the transmission performance significantly regardless the transmitter is operated with positive or negative chirp. The SPM-induced effective $alpha$ -parameter can be used as a design parameter for the high speed (10 Gbps) transmission systems.