A quartz crystal nanobalance (QCN) biosensor was developed for the selective determination of phenylalanine (Phe) in aqueous solutions. A Phe imprinted copolymer was synthesized using polyacrylonitrile and acrylic acid [poly(AN-co-AA)]. The copolymer was then coated on quartz crystal electrode to form complementary structures for the template recognition of Phe. The composite electrode was then used to determine Phe levels in solution. Determinations were based on frequency shifts of molecularly imprinted polymer (MIP) modified quartz crystal electrode caused by Phe adsorption. The frequency shifts were linearly dependent on Phe concentration over the range $50{sim}500;mgL^{-1}$. The results obtained show that the imprinted poly(AN-co-AA) modified biosensor had higher sensitivity (0.5839 Hz/$mgL^{-1}$) than a non-molecularly imprinted copolymer (0.2724 Hz/$mgL^{-1}$). Furthermore, good reproducibility, R.S.D. = 1.84% (n = 7) was observed, and the detection limit was $45;mgL^{-1}$. The selectivity of the imprinted poly(AN-co-AA) modified biosensor was examined using a number of analytes similar to Phe, i.e., dopamine (DA), ascorbic acid (AscA), vanillylmandelic acid (VMA), uric acid (UA), tryptophan (Trp), and tyrosine (Tyr), and the results obtained showed a size dependent selective effect.