We report the controlled synthesis of an amino acid based polymer by reversible addition-fragmentation chain transfer ( RAFT) polymerization of N-acryloyl-L-phenylalanine (A-Phe-OH), which has a carboxylic acid group. Three chain transfer agents (CTAs), benzyl dithiobenzoate (CTA 1), benzyl 1-pyrrole-carbodithioate ( CTA 2), and O-ethyl-S-(1-phenylethyl) dithiocarbonate ( CTA 3), were compared for the direct polymerization of A-Phe-OH without protecting chemistry. With 2,2'-azobis( isobutyronitrile) as an initiator, the dithiocarbamate-type RAFT agent ( CTA 2) is efficient for the preparation of poly(A-Phe-OH) with relatively narrow molecular weight distributions. The effects of several parameters, such as solvent, temperature, CTA to initiator molar ratio, etc., were investigated in order to determine the conditions, leading to optimal control of the direct polymerization. Good control of the polymerization in the presence of CTA 2 in methanol was confirmed by the formation of narrow polydispersity products and the linear relationship between the molecular weight and conversion. Depending on the monomer/CTA ratio, the amino acid based polyelectrolytes, poly(A-Phe-OH)s, with number-average molecular weights between 6500 and 54 400 and polydispersities between 1.23 and 1.27 were obtained, as evidenced by size-exclusion chromatography of the methylated samples.