In order to evaluate the size effect on the mechanical properties of mortar and to improve the size effect theory of mortar, size effect behaviours of mortar were investigated by using test method and theoretical analysis method. Based on the uniaxial compression test on 12 groups of mortar cube specimen and 12 groups of motar plate specimen, the size effect behaviour of mortar specimen and the relationship between mortar strength grade and size effect of compressive strength were investigated. The results indicate that the mortar compressive strength decreases with the increase of the specimen size, while the degree of size effect is enhanced with the increase of mortar strength grade. The energy release based size effect law was introduced to study the size effect of mortar geometrically similar specimen. The size effect of the mortar specimen of non-geometric similarity was also analyzed by correcting the Bazant size effect law with adding intensity term. Finally, the mortar size effect laws which can predict the size effect behavior of geometrically similar specimen, as well as the non-geometrically similar specimen，are derived，respectively. The predicted results of the proposed size effect law match well with test data. The research results provide an important basis for the establishment of the fine microscopic model of masonry, and also provide guidance for the engineering application of mortar.