In the present work, ZnO nanoparticles were synthesized via a simple wet chemical (hydrothermal) technique using zinc acetate dihydrate (Zn(CH3COO)2.2H2O) as a row material dissolved into absolute methanol and aqueous NaOH as precipitation agent. The X-ray diffractometer (XRD) patterns showed a standard hexagonal wurtzite structure of ZnO NPs with average particle size of about 28±2 nm. High resolution-transmission electron microscopy (HR-TEM) micrograph of ZnO NPs showed semi-spherical particles with an average size of about 25.3±6 nm, which agreed well with the XRD results. UV-Vis spectrum of the ZnO NPs revealed the highest absorption band at 366.4 nm and the optical energy gap (Eg) was about 3.12 eV. The Photoluminescence spectrophotometer (PL) spectrum emission illustrated lower UV emission than the visible emission. This indicated to the high defect density in ZnO sample. The effect of concentration of cost-effective Rhodamine B dye on the performance of dye-sensitized solar cells) DSSCs) based on ZnO NPs has been studied. The best performance among the five concentrations was 0.7 mM. The photon to electric energy conversion efficiency of the DSSCs has been found to increase by eight times from 0.07 to 0.58 %. Transient open-circuit voltage decay (TOCVD) method has been used to investigate the electron lifetime (τn) and the electron recombination velocity (Krec) in DSSCs, by using the double exponential function. The results revealed that the Rhodamine B dye can be an efficient dye as a low-cost sensitizing dye for DSSCs.