In this research, synthesized coumarin derivatives were evaluated as antifungal drugs. Scanning electron microscopy (SEM) and transmittance electron microscopy (TEM) techniques were used to investigate morphological changes of fungal cells. In the cells treated with subinhibitory concentrations of compound 61, cell wall was rugged. However, the compound 23 and AmB groups had very few cells with a wrinkled surface. TEM results showed changes in vacuoles of the cells treated with coumarins. Our findings suggest that this increase is due to the phosphodiesterase (PD) inhibitory activity of coumarins.