Over the past decade, several natural and synthetic cationic polymers have been utilized for gene delivery into cells.  Among them, polyethylenimine (PEI) was used for gene therapy successfully.  The present study investigated the effect of PEI and ultrasound waves on ssDNA delivery into saffron cells.  Gel retardation, dynamic light scattering (DLS) and scanning electron microscopy (SEM) assays were employed to determine the physicochemical properties of PEI/f-DNA  polyplex (complex of PEI and  fluorescently labeled DNA).  Moreover, the cytotoxicity of PEI, PEI/f-DNA polyplex and ultrasound were investigated on saffron cells at different concentrations.  The gel retardation results indicated that the formation and neutralization of the PEI/f-DNA polyplex were completed at N/P=5.  The particle size distribution of the polyplexes was from 50 to 122 nm.  The experimental results revealed that the cytotoxicity of the PEI/f-DNA polyplex was lower than that of PEI alone, hence the cells showed both dose- and exposure duration-dependent responses.  Furthermore, the viability of saffron cells declined extremely after 5 and 10 min sonication but this reduction was not significant at 2 min exposure duration.  The results also indicated that the combined utilization of ultrasound and PEI nanoparticles increased the transfection efficiency of saffron cells up to two times higher than those obtained by PEI or ultrasound separately.