Detection of delamination defect in glass fiber reinforced plastics (GFRP) by using ultrasonic testing has been a challenging task in industry. The properties of the constituent materials, fiber orientation and the stacking sequence of laminated composite materials could cause high attenuation of ultrasound signals. Ultrasonic testing is based on the interpretation of the reflected ultrasound signals when a transducer imposes ultrasound waves (pulse) to a material. It is difficult to differentiate if the reflected signal is induced from the defects, fiber content or the intermediate layers of GFRP composites. Most of the time, the drastic attenuation of signals could enshroud the modest changes in the reflected signals from defects. The purpose of this paper is to investigate the influence of fiber orientation, thickness and delamination of GFRP composites on the rise time, pulse duration and attenuation ratio of the reflected ultrasound signal. The rise time, pulse duration and attenuation ratio of A-scan data was observed with respect to different positions of damage (delamination), thickness and stacking sequence of the lamina. It is essential to identify the significant factors that contribute to the abnormal characteristics of the reflected signals in which the defect is identified. Moreover, this paper presents the application of Taguchi method for maximizing the detection of defect in GFRP composites influenced by delamination. The optimum combination of the significant contributing factor for the signal’s abnormal characteristics and its effect on damage detection was obtained by using the analysis of signal-to-noise ratio. The finding of this study revealed that delamination is the most influential factor on the attenuation ratio.