Summary: This study aims at the effect of Zinc ferrite nanoparticles of particle size (14 AE 2) nm prepared by sol-gel technique on chlorella pyrenoidosa at two different concentrations (1.0 mM and 2.0 mM). The structural characterizations of the prepared sample (ZnFe 2 O 4 ) were carried out by XRD, TEM, and XRF. XRD analysis revealed single phased cubic spinel structure having space group Fd3m. XRF analysis confirmed the purity of the sample. The negative effect of ZnFe 2 O 4 on chlorella pyrenoidosa was manifested by the progressive depletion of cell count. It was found that the algal growth was inhibited by 47% even at a low concentration of 2 mM indicating that zinc ferrite nanoparticles are promising agent for the control of unwanted algal growth. To the best of our knowledge, the effect of ferrite nanoparticles on algae has not been reported elsewhere.