We developed a novel Torenia (Torenia fournineri) mutant, frilly petal undulation1 (fpu1), with serrated and wavy petals by irradiation of heavy-ion beams to dry seeds. Unlike previous findings where cytokinin induced serrated petal tips, fpu1 showed genetic mutations leading to both serrated edges and pronounced petal undulation. Moreover, the fpu1 leaves resemble the wild-type ones, indicating that the fpu1mutation affects only to the floral phenotype. In the fpu1 petals, the size of adaxial cells was larger than that of abaxial cells, in contrast to wild-type plants where no difference in cell size was observed between these surfaces. The joint point of vascular bundles in fpu1 petal were 1.5 times denser than wild-type plants. These observations suggest that the undulation of the fpu1 petals may be due to the difference in vascular bundles and cell size. Crossing fpu1 with wild types produced progenies with varied phenotypes: 37 resembled the wild type, 81 showed intermediate serrated petal tips, and 38 plants exhibited the distinct fpu1 frilled petals, following a 1:2:1 segregation ratio (χ2 = 0.89). This result suggested that the fpu1 mutation is semi-dominant and induced in a single locus. The bulk DNA extracted from individual fpu1-like progenies was sequenced and mapped to the genomic sequence, revealing three homozygous mutations in two distinct regions of the same 8-Mbp contig. This contributes to our understanding of the genetic factors influencing petal morphology in Torenia.