Based on the dynamics of charged particles in plasma with the two-fluid description, the equation for magnetic field generation and maintenance is derived. The nature of magnetic field in cosmic plasma and the generation can be understood from the equation. The ions and electrons are considered as two independent fluids with the collision connected. Due to the different viscosity between ions and electrons, there will be a velocity difference in ions and electrons. This causes a current in the plasma and generates magnetic field. IN the plasma system, the differential velocities and the viscous forces of the electrons are the source of magnetic field.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html