Decays of Λ in nuclei, nonmesonic mode, are studied by using the ΛN → N N weak transition potential derived from the meson exchange mechanism and the direct quark mechanism. The decay rates are calculated both for the Λ in symmetric nuclear matter and light hypernuclei. We consider the exchange of six mesons (π, K, η, ρ, ω, K * ). The form factor in the meson exchange mechanism and short range correlation are carefully studied.