The purpose of this study was to investigate the effects of energy of virtual monochromatic images (VMIs) and the strength of iterative reconstruction on computed tomography (CT) number, image noise, and visual evaluation. Low keV VMIs improve the CT number of the contrast agent but also increases image noise, making them difficult to use. However, the adoption of iterative reconstruction has made low keV VMIs possible to use. We scanned a phantom containing dilute contrast agent with single energy (SE) and dual energy (DE) scanned images, comparing CT number, image noise (SD), and the contrast-to-noise ratio (CNR). The CT number and SD of 40 keV VMIs were 2.8 and 2.4 times higher, respectively, than those of 120-kV CT images. VMIs showed an inferior CNR compared with SE images due to increased image noise. The higher the strength of iterative reconstruction, the higher the visual evaluation score. Visual evaluation scores were highest at 50 keV. Low-keV VMIs are greatly affected by increased image noise. But by taking into the effects of parameters such as tube voltage and strength of iterative reconstruction, low keV VMIs can improve the visualization of blood vessels and reduce the use of contrast agent.