It is theoretically proved that the Practical laser linewidth limit (PLLL) is much larger than the theoretical laasr linewidth limit (TLLL) given by Shawlow-Townes formula. PLLL depends not only on the resonator optical path length flucturation (ROPF) but also the resonator round-trip time. Therefore, the ROPFs effect on the linewidth of a semiconductor laser with short round-trip time is less than a gas or solid laser with long round-trip time.