A focus adjusting mechanism (50) for use in a portable electronic device (100) with a lens module (40) is provided. The lens module includes a lens barrel (42) and a lens barrel holder (44). The lens barrel is threaded into the barrel holder. The focus adjusting mechanism includes a driving member (52), a follower (54) and a transmission rod (56). The driving member has a plurality of circumferential teeth (5242) formed thereon. The follower has a plurality of linear teeth (5472) formed thereon. The linear teeth engage with the circumferential teeth for transforming the rotation of the driving member to the linear movement of the follower. The transmission rod is disposed between the follower and the lens barrel to transform the linear movement of the follower into the rotation movement of the lens barrel relative to the lens barrel holder.