This paper deals with the simultaneous determination of four amino acids, namely tyrosine (tyr), tryptophan (trp), phenylalanine (phe) and 3,4-dihydroxyphenylalanine (di-phe), by direct UV spectrophotometry combined with the Kalman filter algorithm. In 0.1 mol l-1 of hydrochloric acid solution, the UV spectra of these four amino acids overlap severely. The Kalman filter algorithm was applied to resolve this problem. All the spectral data were collected on the hard disk of the IBM PC/AT microcomputer which was connected to the VARIAN DMS 200 UV Visible spectrophotometer via IEEE-488 interfaces. Spectral data were processed from 213.1 nm to 281.8 nm every 0.3 nm, so 230 data points were treated. Eighteen synthetic mixture samples were determined by the proposed method. The mean recoveries were 98.85% for tyr, 99.91% for trp, 100.4% for phe, and 101.4% for di-phe. The relative standard deviations (RSD) of recoveries were 2.87%, 2.35%, 3.15% and 1.88%, respectively. These results are compared with those obtained by target transformation factor analysis (TTFA) method.