"Co-precipitation" is one of the most popular methods for preparation of magnetic iron-dextran particles,which is simple and easy to operate. These particles can be obtained by dissolving FeCl₂,FeCl₃ together with dextran in H₂O in a proper ratio and adding NH₄OH into this solution. After the chemical reaction completed,the poduct is dark-brown including numerous different sized magnetic iron-detran particles and their aggregates due to the effects of temprature,pH,the speed of NH₄OH addition and some other factors. In fact,the primary reaction product is centrifuged to eliminate the large particles and to obtain ideal-sized particles,while this centrifugation results in the loss of the most primary reaction product and low production of the ideal-sized particles. In this experiment,the pellet from the centrifugation was treated with ultrasound,and as the result,the production of magnetic iron-dextran particles was increased to 20 times of the original production from 0.5 g to 10 g. When these particles were added to the mixtures of singel cells and conjugating pairs of Paramecium caudatum,highly synchronized conjugating pairs were obtained,which indicates its high quality of the particles. The importance of the current improvement for preparation of magnetic iron-dextran particles can be evaluated by its universal use in bioscience,biomedical and bioengineering due to their excellent biocompatibility and targeting performance.