Biological effects of millimeter waves (MMWs) at the cellular level are explored in experiments using culture dishes containing cell monolayers with MMW irradiated from the underneath. Analysis is carried out for culture dishes with diameters much larger than the MMW wavelength to discover the relation between the portion of the incident MMW power density (PD) irradiated into the cell monolayer and the interfering factors such as the bottom thickness of the culture dish, the MMW wavelength, and the electromagnetic parameters of the dish and culture solution. With the help of the finite-difference time-domain (FDTD) technique, another analysis is performed to display the effect of the dish geometries upon the MMW PD irradiated into the cell monolayer in a specific culture dish with a diameter similar to the MMW wavelength. The results indicate that rigorous analysis, precise measurement and accurate calculation of MMW PD are essential accompaniments for these experiments, and large-diameter culture dishes are more preferred in experiments to small-caliber ones.