Magnetic suspensions have a lot of applications in biology, medicine and technology [1]. Magnetic and mechanical properties of these liquids depend on interaction between individual magnetic particles in suspension. Such magnetic interaction forces between particles are very small being of about 100 fN [2]. These magnetic force values are comparable with Brownian stochastic forces exerted to these particles. This fact complicates direct measurement of forces between microparticles. However the presence of weak magnetic forces considerably changes statistical properties of particles Brownian motion. Optical tweezers technique combined with correlation analysis of Brownian motion allows measuring forces between individual particles. This approach was used for detection of hydrodynamic interaction between microparticles [3]. In this work Brownian motion of composite polystyrene 3 μm spheres with paramagnetic magnetite nano grains was studied. Significant changes of the correlation function of the particles Brownian motion were observed in case of the presence of external magnetic field comparing to the case when magnetic field was absent which is shown in the figure. Round dots correspond to the correlation function in case of magnetic field absence. Correlation function obtained in case of presence of magnetic field is shown by square dots. Curves show theoretical approximation of correlation functions. Magnetic interaction between particles increase the correlation functions value. Theoretical model based on magnetic dipole interaction between particles approximation was suggested. Experimental results are in good agreement with theoretical model. Approximation of the experimental results with the theoretical expression for particles Brownian motion correlation function allows one to obtain the value of magnetic forces between particles and their magnetic moments.figureFig. 1. The correlation functions of magnetic particles Brownian motion in external magnetic field. Suggested method based on nanometers displacement detection allows measuring weak magnetic forces with an accuracy of femtoNewtons and magnetic moment which appeared to be of about 10−14Am2 . This technique can be useful for micro scale magnetic properties characterization of particles suspensions used in medicine or technical devises. [1] O. Philippova, A. Barabanova, V. Molchanov, A. Khokhlov. Eur. Polym. J. 47 (2001) 54 [2] L. E. Helseth. J. Phys.D 40 (2007) 3030. [3] S.-C. Meiners, S. R. Quake. Phys. Rev. Lett. 82 (1999) 277.