  Wave-particle duality of photons with losses in the Mach-Zehnder
interferometer (MZI) is investigated experimentally and theoretically. The
experiment is done with the standard MZI with the beam splitter or the beam
merger being continuously varied. The losses are deliberately introduced either
inside the MZI (the two arms between the beam splitter and beam mergers) or
outside the MZI (after the beam merger). It is proved that the unbalanced
losses have great influence on the predictability $P$ (particle nature) and
visibility $V$ (wave nature). For the former case the duality inequality holds
while for the later the duality inequality is ``violated''. We get $P^2+V^2>1$.
This ``violation'' could be eliminated in principle by switching the two paths
and detectors and then averaging the results. The observed results can be
exactly explained theoretically. The experiment is done with coherent beam,
instead of single photons, and we have proved that they are exactly equivalent
in duality experiment with MZI.
