A nozzle includes an expansion portion and a coupling portion having an inner end, an outer end, and an air hole extending from the inner end through the outer end. A connecting portion is connected between the expansion portion and the inner end and includes an opening intercommunicating with the air hole. The nozzle is coupled with a neck of a balloon. The expansion portion spreads the neck. The opening is received in the neck. The air hole intercommunicates with an inflation space defined in the balloon when a gas with a sufficient pressure is filled into the air hole. The air hole does not intercommunicate with the inflation space when no gas with the sufficient pressure is filled into the air hole.