An engine clutch transfer torque determining apparatus of a vehicle includes: a power source including an engine and a driving motor; an engine clutch that is located between the engine and the driving motor to selectively connect the engine to the driving motor; and a vehicle controller that controls release or engagement of the engine clutch to implement a driving mode, wherein the vehicle controller releases a clutch hydraulic pressure in the engine clutch when a running state of the vehicle satisfies a transfer torque determining advancing condition, controls a speed of the motor and a speed of the engine while maintaining a speed difference between the motor and the engine in a predetermined relative speed range, and determines a transfer torque of the engine clutch based on a clutch hydraulic pressure and a clutch input torque when the engine clutch slips.