Tachyon condensation in a single unstable D-brane is discussed in the framework of boundary state formulation. The boundary state in the background of the tachyon condensation and the NSB-field is explicitly constructed. We show in both commutative theory and noncommutative theory that the unstable D-branes behaves like an extended object and eventually reduces to the lower dimensional D-branes as the system approaches the infrared fixed point. ©2002 American Institute of Physics.