Diamond crystal-based detector for neutron flux measurement is a promising solution as a fast, small and stable detector especially suited for fast transient measurements. The work aims to test the method and to find the best parameters of the pulse shape analysis for the thermal neutron detection. The developed diamond detectors were tested with the AmBe neutron source and in the VR-1 reactor core. Based on the results obtained from the AmBe measurements, the discrimination conditions for the pulse shape analysis were set and optimized. The diamond detector was then tested in mixed n-γ field of the nuclear reactor core. First results from the VR-1 reactor measurement showed high yield of thermal neutron interactions (up to 33% of all recorded interactions, like in AmBe measurements). The presented results therefore proved that diamond detectors are promising for the fast transient measurements.