We describe the design and performance of modular jack type mini-MT connectors that employ a compact 2-fiber ferrule in a newly designed housing and adaptor. The basic mechanism for coupling the housings is similar to that of a conventional electric modular jack type connector. Our connectors have a locking mechanism to prevent accidental disconnection and a high packaging density of 2 fibers/cm2. Fabricated connectors for single-mode 2-fiber ribbons exhibit a low insertion loss of 0.18 dB and a high return loss of more than 40 dB using a flat endface ferrule with protruding fiber finish. In addition, they have an insertion loss of 0.20 dB and a return loss of 60 dB when an angled endface ferrule is used. We have confirmed that these connectors are highly effective for use in the duplex connection of advanced optical networks.