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, 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 of a fluid under reaction and/or passing through the fluid paths as well as spatial distribution changes of the sample fluid species in the space between the central and outer conductors.