Background: sysmex XT 1800i coulter is a hematology analyzer incorporating new electronic and mechanical design with advanced algorithm technology to perform CBC. The principles of sysmex XT1800 are electrical impedance technology, optical scatter technology, flow cytometry. The sysmex XT-1800i haematology  analyser is used in unique fluorescence flow cytometry (FFC) technology. FFC looks at deoxyribonucleic acid (DNA) content, cell size and inner cell complexity rather than cell size alone. This generates remarkably accurate results. Review rates and turnaround time are reduced due to specific fluorochrome labelling. The XT-1800i offers true quantitative immunoglobulin ( IG) counting instead of the flagging delivered by other technologies.  Patiant and Method: Evaluation of this instrument was performed on 200 samples of inpatient and outpatient people in Ain Shams University Hospitals. We collected samples from all departments of the hospital randomly of any age and sex except those of age less than 18 years old. Results : this study was centered upon RBCs indices which were  mean corpuscular volume (MCV),mean corpuscular hemoglobin(MCH),red cell distribution width (RDW) after comparing sysmex XT 1800 coulter results  for RBCs parameters with  morphology  by smear review under the microscope. We found that the device sensitivity was 74.7%  , 75 % ,83 % and specificity was 74.3% , 79.5 % ,78.7%  according to RBCs size ,chromasia and anaesocytosis respectively. Conclusion: at the end of our study we reached to specific criteria at which we must do a blood film smear review to evaluate RBCs abnormalities which cannot be evaluated by the device.