A modifying agent, a modified conjugated diene polymer prepared using the same, a method for preparing a modified conjugated diene polymer, and a rubber composition including a modified conjugated diene polymer are disclosed herein. In some embodiments, a modifying agent comprises a compound represented by formula 1. The modifying agent is capable of improving processability of a rubber composition including the polymer prepared using the modifying agent. Fuel consumption properties of a car using tires including the rubber composition may be improved.