The modified ABC model explains the floral morphology of many monocots, such as the lily and tulip, in which the perianth consists of two layers of almost identical petaloid tepals. According to the modified ABC model, B-class genes are expressed in two perianth whorls, inducing the petaloid structure in both whorls 1 and 2. In this study, we analyzed the expression and function of the B-class genes in the grape hyacinth (Muscari armeniacum). We isolated two DEFICIENS (DEF)-like genes (MaDEF1 and MaDEF2) and three GLOBOSA (GLO)-like genes (MaGLOA1, MaGLOA2, and MaGLOB) from M. armeniacum using rapid amplification of cDNA ends (RACE). Expression analysis showed that MaDEF1 and MaDEF2 were expressed in whorls 1, 2, and 3, whereas MaGLOA1, MaGLOA2, and MaGLOB were expressed in all four whorls. These results support the modified ABC model in M. armeniacum. Overexpression of MaGLOA1 and MaGLOB in Arabidopsis thaliana resulted in a morphological change of sepals to petaloid structures in whorl 1, indicating that the function of these genes is similar that of the B-class orthologs PISTILLATA and GLO in A. thaliana and Antirrhinum majus, respectively. In addition, yeast two-hybrid assays revealed strong protein–protein interactions between MaDEF1 and MaGLOA1, suggesting that MaDEF1–MaGLOA1 is likely to have the main B-function in M. armeniacum. These data support the modified ABC model in M. armeniacum.