An ethyl methane sulphonate (EMS) induced narrow leaf pepper mutant, Canl(Capsicum annuum narrow leaf), was phenotypically characterized using the wild-type Zhangshugang pepper(WT) as a control. The mutant exhibited significant reductions in leaf width, leaf area and leaf epidermal cell size, whereas no significant difference was observed in leaf length and epidermal cell numbers compared to the wild type. Genetic mapping populations, F1 and F2, were constructed from a cross between WT and mutant Canl, and the number of normal-leaf plants and narrow-leaf plants in each population was investigated. Analysis of F1 and F2 populations based on genetic laws suggested that narrow-leaf trait is controlled by a single recessive nuclear gene. Fine-mapping of the leaf morphology gene was performed using BSA-Seq and KASP, the target locus was delimited to an interval between SNP14074961 and SNP14299292 on chromosome 6. Within this candidate region, one candidate gene Caz06g08110, was identified. Transcriptome analysis of leaves from WT and mutant Canl revealed that differentially expressed genes were mainly enriched in phytohormone signal transduction pathways, and genes involved in auxin and cytokinin signaling exhibited significant changes in their expression levels. We hypothesize that the Caz06g08110 gene modulates leaf phenotype by influencing plant hormone signal transduction.