The investigation was aimed to quantify the Gallic acid present in Lagerstroemia speciosa leaves (Lythraceae). The High-Performance Thin Layer Chromatography (HPTLC) quantification was performed for acetone (AE), methanolic (ME) and chloroform (CE) extract of leaves of L. speciosa. The pre-coated silica gel 60 F254 was used for complete separation of compounds using the mobile phase pet. Ether: ethyl acetate: formic acid (5:5:1 v/v).The validation of the extracts was carried out using ICH guidelines for precision, repeatability and accuracy showing the Rf 0.49 against standard Gallic acid. Linearity range for Gallic acid was done from 200 to 1000 ng/spot (AE) and200 ng to 600 ng/spot (ME), with Correlation, coefficient r = 0.99 (AE) and 0.54 (ME) in the said concentrations. The composition in crude leaf extract was determined to be of 49.712 mg (AE) and 20.125 mg (ME), while it was not found in chloroform extract against standard Gallic acid. Hence the proposed method was very simple, precise, accurate and easy for the screening of the bioactive compounds present in the acetone and methanolic extracts of the leaves of L. speciosa. It was observed that the acetone extract subjected to cytotoxicity showed promising activity at higher concentrations (100 and 200 μg/ml) showed 92.9% and 87.13% inhibition against MCF-7 cell lines respectively. The photocatalytic activity of the acetone and methanolic extracts of methyl orange was found to be 90.25% (190 min) and 89.03% (180 min) respectively. Therefore this can be used as an indicator of purity of herbal drugs and formulation containing L. speciosa. © 2017 Elsevier B.V.