A yarn feeder for flat knitting machines is located on a yarn feeder guide track to move transversely thereon and includes a baseboard slidably coupled on the yarn feeder guide track, a first rocking arm, a second rocking arm and a yarn feeding unit located on the baseboard. The first and second rocking arms have respectively a first pivot end and a second pivot end coupled with the baseboard, and a first pressed end and a second pressed end to receive a horizontal force to swing respectively about the first and second pivot ends serving as fulcrums. The yarn feeding unit includes a yarn feeding arm located between the first and second rocking arms, and a first linkage bar hinged on the yarn feeding arm and first rocking arm to move simultaneously, and a second linkage bar hinged on the yarn feeding arm and second rocking arm to move simultaneously.