The real part of the dielectric function ( 1 (w)) is related to the energy stored by the medium when a material is exposed to an electromagnetic spectrum. Figure depicts the calculated 1 ( ) as a function of energy for the p-BCN monolayer at strain-free and strain applied conditions. For strain-free monolayer, two sharp peaks are located at VR (1.62 eV-3.23 eV or 360 nm-740 nm) while multiple peaks in the UV region ultimately become steady beyond 16 eV. After compressing and stretching, the intensity of peaks increases slightly anyway. This unusual result is described by the Penn model, which is also observed in an earlier report . This phenomenon is also attributed to the geometrical re-arrangement of the pentagonal 2D system. Additionally, the value of 1 ( ) is positive throughout the whole range of photon energy in both strained and strainfree conditions. The absence of negative 1 ( ) confirms that there is no plasma frequency, the negative value of 1 ( ), indicating the persistent semiconducting behavior of monolayer. Furthermore, the value of 1 ( ) at zero photon energy, which corresponds to the static dielectric function 1 (0), is 2.04 at equilibrium condition. The static dielectric constant value of p-BNC monolayer is very close to that value of penta-graphene but smaller than that of well-known 2D materials including h-BN for E||x. However, this value slightly changes with applied strain. Interestingly, at 6% of compressive (tensile) strain, it reaches up to 2.14 (2.08) thereby alerting the fact that p-BCN is mechanically unstable at 6% of compression.