Background and Objectives: Magnetic fluids are unique nanodispersed systems that have the properties of a magnetic material and a liquid. Although their magnetic properties are inferior to steels and ferrites, they are supermagnetic compared to most liquid media. When a magnetic field is applied to a magnetic fluid, agglomerates of ferromagnetic nanoparticles are formed in it, which were visually observed in this work. Materials and Methods: The dependence of the modulation depth of laser radiation with wavelengths of 450 nm, 550 nm and 650 nm on the concentration of nanotubes and the magnitude of the magnetic field induction is studied. The transmittance of polarized radiation in the optical range is measured depending on the relative position of agglomerates of ferromagnetic nanoparticles and the vector of the electric field strength of laser radiation. The angle between the electrical component of the laser radiation and the magnetic field induction vector changes by rotating the laser diode and polarizer with a servomotor relative to the vertical axis. Results: The maximum value of the modulation depth for magnetic fluid without carbon nanotubes is 15% and is observed for laser radiation with a wavelength of 650 nm. Adding multi-walled carbon nanotubes to the magnetic fluid makes it possible to increase the modulation depth by approximately one and a halftimes. Conclusion: It has been revealed that with increasing wavelength of laser radiation, the modulation depth increases. As the magnetic field induction value increases, the modulation depth of polarized laser radiation increases for all wavelengths. The addition of carbon nanotubes to the magnetic fluid leads to their alignment along the agglomerates.