We have measured the angular correlation of the pair nucleons np and nn emitted from the nonmesonic weak decay (NMWD) of12ΛC produced via the (π+, K+) reaction in coincidence measurement. The Λ p → n p and Λ n → n n modes were clearly identified by measuring the back-to-back correlation of the emitted nucleon pairs which is the characteristic of two-body kinematics. From the measured nucleon pair numbers Nn nand Nn p, the ratio Γn/ Γpof the partial decay widths Γn(Λ n → n n) and Γp(Λ p → n p) of12ΛC was extracted to be 0.51 ± 0.13 (stat) ± 0.05 (syst); this result is almost free from the ambiguity due to the nuclear final state interaction and 3-body decay process, which were inherent in the previous results. The obtained Γn/ Γpratio of12ΛC (p-shell) is close to that of5ΛHe (s-shell). The results are consistent with those of recent theoretical calculations. © 2006 Elsevier B.V. All rights reserved.