Filament wound composite pressure vessels are widely used as rocket motor case for space applications due to its high performance factor. High performance factor is attributed due to the fact that composite materials are having higher specific strength, high specific modulus and strength tailorability characteristics which results in reduction of weight of the structure. However composite structures are more prone to damage accumulation such as matrix cracking, delamination, de-bonding, porosity, fibre buckling, fiber breakage etc. which are encountered in entire manufacturing cycle. Timely and accurate detection of above damage throughout its service life is necessary to have a reliable structure. Here, a through transmission ultrasonic testing methodology has been established for detection of defect in large sized composite pressure vessel. In the last decade, many open literatures have been available on design, development and qualification of composite pressure vessels, however no comprehensive work is available on structural integrity assessment using above said technique. This paper discusses on insight of various defect arised due to process parameter variation during manufacturing and its detection using through transmission ultrasonic technique. Further challenges multifold due to lack of full proof methodology and reference standards in open source. Hence customized reference blocks have been made simulating defect such as delamination & simulating varying thickness and their ultrasonic responses have been characterized. Ultrasonic response on full scale composite pressure vessel was studied and finally an overview of structural integrity assessment of composite pressure vessel has been presented.