A method and a system of diagnosing a vehicle oil leak are capable of detecting an oil leak state of transmission oil on the basis of logic. The method includes an operating condition determination step of determining, by a controller, whether a minimum operating condition of a vehicle is satisfied. The minimum operating condition is required to normally generate a target line pressure by operating an EOP. The method includes a counting step of counting, by the controller, an operating time of the EOP when the minimum operating condition is satisfied and includes an oil leak diagnosis step of diagnosing, by the controller, an oil leak when the counted continuous operating time of the EOP is a permissible time or more.