A transmission control method for HARQ is provided for improving HARQ performance in a mobile communication system. The transmission control method for Hybrid Automatic Repeat reQuest (HARQ) in a mobile communication system according to the present invention includes receiving an downlink resource assignment message; determining whether configured downlink assignment has been indicated to an HARQ entity since a previously received downlink assignment for a User Equipment's (UE's) Cell-Radio Network Temporary Identifier (C-RNTI) for the same HARQ process; maintaining, if the configured downlink assignment has not been indicated to the HARQ entity, the HARQ process; and processing, if the configured downlink assignment has been indicated to the HARQ entity, the downlink resource assignment message as a resource assignment message for initial transmission.