Disclosed is a probe unit for nuclear magnetic resonance. The probe unit includes a cylindrical outer conductor, a cylindrical central conductor concentrically disposed in the outer conductor, a membrane assembly and fluid paths guiding the flow of fluid in the space between the central and outer conductors. By applying a RF current between the outer and central conductors, a sensing magnetic field is generated in a radial direction. Accordingly, it is possible to detect in real time and in situ, variations of characteristics as well as spatial distribution changes, of a fluid under reaction and/or passing through the fluid paths in the space between the central and outer conductors.