An ice maker of a refrigerator includes a cooling unit for generating cold air; a case mounted in a food storage space, and defining a cooling space for receiving the cold air; an ice making assembly for making ice; a rib unit provided at one side of the ice making assembly; and a bucket arranged at one side of the ice making assembly for receiving the ice. The ice making assembly includes an ice tray arranged in the cooling space, the ice tray having an ice making recess formed in an upper surface thereof for making ice; and a cold air guiding unit arranged at a lower side of the ice tray for guiding the cold air to the lower side of the ice tray, and a rib unit including cooling ribs protruding downward from a bottom surface of the ice tray while extending in a state of being rotated.