We have measured the branching ratio of the pi(-) mesonic weak decay a
nd the weak decay lifetime of(Lambda)( 12)C with the use of the SKS sp
ectrometer at KEK 12 GeV PS. The pi(-) mesonic decay width (Gamma(pi-)
) was derived and compared with theoretical calculations. The measured
Gamma(pi-) is consistent with the theoretical calculation including t
he distortion effect of the pion wave function.