Abstract: In this article, achievements about S4-gene between self-compatible and self-incompatible Japanese pear cultivars last 20 years were summariaed. The results were show as follow: Nijisseiki(S2S4) is a self-incompatible cultivar, and its self-compatible mutant, Osa-Nijisseiki (S2S4SM, SM=stylar part mutant ) of Japanese Pear. The reason of the self-compatibility of Osa-Nijisseiki is considered as low expression of the S4SM-gene and the S4SM-gene expression only in stigma. and late expression of the S4SM. Osa-Nijisseiki is a variant of Nijisseiki in which the S4-allelic gene in the S-locus is exclusively mutated or deleted, causing severely impaired or suppressed expression of its gene product, S4-RNase, and the style. The amino acid sequence of S4-protein was 228 residues long, which comprised a 27-residue signal peptide and a 201-residue mature protein. S4-gene was composed of 997 bp including a 684 bp open reading frame of positions 85-768, there were two candidates for the putative polyadenylation signal of positions 925-931 and 943-950. S4-protein showed inhibitory effects on the pollen germination and the pollen tube growth.