A rapid, precise and selective method is described for determination of penicillin derivatives - Penicillin derivatives are separated from closely related degradation products on reversed phase C18thin layer by using solvent system methanol - aqueous KH2PO4acetonitrile (8:4 : 1). The pH was adjusted to pH 4.1 ± 0.05. The plates were air dried and measure on spectrodensitometer at 230 nm wavelength. The results of analysis show good agreement with official methods.