Glass fibre reinforced polymer (GFRP) composites are being increasingly used as structural sections owing to their high strength-to-weight ratio and corrosion resistance, making accurate determination of their tensile properties essential for safe and reliable design. This study presents a comprehensive comparison of the tensile behaviour of pultruded GFRP composites using ASTM D3039 flat specimens and ASTM D638 dog-bone specimens. The effects of specimen geometry on tensile properties and failure characteristics were systematically investigated. Fibre characteristics were analysed using scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and heating to extract the fibres, while digital image correlation (DIC) was employed to monitor full-field strain evolution during tensile loading. Compared to flat specimens, dog-bone specimens exhibited lower tensile strength and elastic modulus of 7-11% and 1-22%, respectively. More importantly, the failure mechanisms differ with flat specimens predominantly failing by longitudinal splitting accompanied by edge delamination, whereas dog-bone specimens exhibited transverse rupture. The results suggest that dog-bone specimens tested as per ASTM D638 would not accurately represent the intrinsic tensile behaviour of pultruded GFRP composites. Therefore, only ASTM D3039 is recommended for tensile characterization, to obtain a reliable basis for quality control and structural design with pultruded GFRP composites.