A novel and rapid method was validated for the determination of phenolics in plants by high-performance liquid chromatography coupled to photodiode array detection. The optimized method was employed for the quantification of phenolic acids and rutin present in Heliotropium thermophilum. Eight samples collected from two locations were analyzed. Additionally, temperature dependent changes in the phenolic composition of H. thermophilum samples was evaluated. A series of standards were used for identification that included gallic acid, protocatechuic acid, 4-hydroxybenzoic acid, chlorogenic acid, vanillic acid, caffeic acid, syringic acid, p-coumaric acid, rutin, ferulic acid, benzoic acid, 2-hydroxycinnamic acid, abscisic acid, and trans-cinnamic acid. Chromatographic separation was performed on C-8 column at 40 degrees C with gradient elution. A phosphate buffer (20mM, pH: 2.5) and acetonitrile mixture was used as the mobile phase with detection at 280nm wavelength and a flow rate of 1.5mL/min. The developed method was shown to be sensitive, accurate, and reproducible for the determination of phenolic acids and rutin in plants.