Weak nonmesonic decay of Lambda hypernuclei is almost unique way to study strangeness changing weak Lambda N --> NN process. The study of branching ratio gave spin-isospin structure of the the nonmesonic decay. A recent experiment, which observed asymmetry of protons from the polarized Lambda hypernuclei, gives the interference term of amplitudes between different parities. It is shown that the asymmetry parameter elucidates the dominant part of the nonmesonic decay mechanism.