Epithelial monolayers play a vital role in gastrulation, tumor metastasis and wound healing, and protect the tissue from pathogens. During these processes, the monolayers sense, generate, and exert mechanical forces to perform their biological functions, but their mechanical properties are rarely known. Here, we use the vertex dynamics models to investigate the mechanical behaviors of an epithelial monolayer and the configurations of the cells within the monolayer during stretch. It was found that the epithelial monolayer exhibited elastic and plastic properties, due to the geometric extension of cells and cell division, respectively. Moreover, the elasticity of monolayers was increased by enhancing the cell adhesion or by reducing the active contractility of actin-myosin rings. This study furthers our understanding of the relationship between the mechanical properties of individual cells and of their monolayers, and may shed light on linking cell behavior to the patterning and morphogenesis of tissues.