•Magnetic separation method on harvesting tiny marine microalgae was developed.•Recovery efficiency of Nannochloropsis maritima reached above 95% within 4min.•Reuse of culture medium from magnetic separation was realized for algal culture. An efficient magnetic separation technology using Fe3O4 nanoparticles was developed for harvesting marine microalgae Nannochloropsis maritima from culture broth. Recovery capacity of these nanoparticles was affected by microalgal growth phase and reached the peak value when the microalgal growth reached its maximal biomass after 18days. The recovery efficiency of microalgal cells from the culture medium reached more than 95% at the particle dosage of 120mg/L within 4min. Electrostatic attraction at acidic pH and cell aggregation under neutral and alkaline conditions was beneficial for harvesting the algal cells. Higher operation temperature resulted in higher adsorption capacity of these nanoparticles for microalgawl cells. Reuse of the culture medium obtained from magnetic separation gave similar biomass production in comparison with that from centrifugation separation after 5 recycles. Together with these results provide a great potential in high-efficient and economical harvesting of tiny marine microalgae using magnetic separation technology in practice.