In the present work, a new theoretical study involving comparative investigation among different PCMs on the performance of conventional solar still has been performed. The most used organic and inorganic PCMs complete with their specified thermos-physical properties with the melting temperature as the main effective properties are studied. In addition, economical studies for all selected PCMs have been performed. Therefore, the finest PCM is selected carefully to satisfy high productivity with low cost. The results show that the inorganic PCM capric-palmatic and organic PCM A48 have the advantages of both high productivity and low cost. However, organic PCM A48 is recommended as it has less negative effect on the environment. In addition, it is found that the thickness of the PCM has no remarkable effect on the productivity and so, the small thickness is recommended to give low cost. The present theoretical study is compared with previous experimental work and decent agreement has been accomplished.