Background: To determine the basic physical and dosimetric properties of a new synthetic single-crystal diamond detector and its application for relative small field dosimetry. Materials and Methods: The pre-irradiation dose required to stabilize detector response, dose rate dependence, photon and electron energy dependence, temperature dependence and angular dependence of MicroDiamond detector response were evaluated. Output factors on Leksell Gamma Knife Perfexion and on CyberKnife were measured to assess detector feasibility in small radiation field dosimetry. For all measurements, the detector was connected to Unidos electrometer set to 0 voltage. Results: Relative output factors measured on Leksell Gamma Knife Perfexion for 4 mm and 8 mm collimators were in agreement with Monte Carlo reference values from the manufacturer, with deviations of 0.3% and 2.1%, respectively. For CyberKnife and fixed circular collimators, the difference in output factor values did not exceed 2% from vendor-supplied values, even for the smallest radiation field with a diameter of 5 mm. Conclusion: Our results indicate that the MicroDiamond detector is a promising tool for relative small field dosimetry. For output factor measurements on Leksell Gamma Knife Perfexion and CyberKnife, the detector can be used with minimal response corrections applied (correction factors not larger than 2%).