Disclosed herein is an apparatus and method for liquefied natural gas (LNG) carrier propulsion. In the apparatus and method, the propulsion of an LNG carrier is done by only a single main diesel engine and has construction to promptly cope with emergencies caused by malfunction of the main diesel engine. The propulsion apparatus for an LNG carrier comprising a boil-off gas re-liquefaction apparatus for re-liquefying boil-off gas generated in LNG storage tanks to return re-liquefied boil-off gas back to the LNG storage tank comprises a single main diesel engine, a propulsion shaft separably connected to the main diesel engine, and an electric motor for propulsion separably connected to the propulsion shaft and supplied with power intended for operation of the boil-off gas re-liquefaction apparatus.