This paper deals with structural shape optimisation of prismatic shell structures using genetic algorithm. In the formulation of the optimisation problem, the minimum value of the strain energy is thought as objective function while the volume of each structure remains constant. The optimisation process is carried out for two structures: cylindrical and folded plate structures. The design variables are chosen such that the shape of each studied structure can be represented. The proposed algorithm, used to generate new structural shapes, is linked to a finite element package to calculate the objective function. It is observed that the proposed optimisation algorithm provides an efficient and reliable way of obtaining better solutions for such class of prismatic shell structures.