This paper describes a reliable method to selectively produce metastable R-form glycine crystals with a controlled size in batch cooling crystallization. Seeding had a large effect both on the polymorphism and the size distribution of the product crystals. In the case of sufficient seed loadings of the R-form crystals, the metastable R-form crystals were obtained selectively as a product. Simultaneously, the crystal size distribution became unimodal with no fine crystals. However, at low seed loadings, mixtures of both the metastable R-and stable γ-form crystals were obtained. The measurement of supersaturation transient suggested that the low supersaturation caused by the growth of sufficient seeds played a key role in suppressing nucleation of the metastable R-and γ-form crystals, whereas, with no seeding, only the stable γ-form crystals were obtained. The seeding technique presented here could be a general and practical method to produce metastable polymorphs with controlled crystal size.