A yarn feeder for flat knitting machines is located on a yarn feeder guide track (10) to move transversely thereon and includes a baseboard (21) slidably coupled on the yarn feeder guide track (10), a first rocking arm (22), a second rocking arm (23) and a yarn feeding unit (24) located on the baseboard (21). The first and second rocking arms (22, 23) have respectively a first pivot end (221) and a second pivot end (231) coupled with the baseboard (21), and a first pressed end (222) and a second pressed end (232) to receive a horizontal force to swing respectively about the first and second pivot ends (221, 231) serving as fulcrums. The yarn feeding unit (24) includes a yarn feeding arm (25) located between the first and second rocking arms (22, 23), and a first linkage bar (241) hinged on the yarn feeding arm (25) and first rocking arm (22) to move simultaneously, and a second linkage bar (242) hinged on the yarn feeding arm (25) and second rocking arm (23) to move simultaneously.