The fluence rate (FR) distribution in an ultraviolet (UV) reactor was determined experimentally in situ by use of a novel 360° micro fluorescent silica detector (MFSD). The UV response of the MFSD was systematically characterized, and the results indicated that this detector responded only to UV in the range from 210 to 280 nm. The nonlinearity was found to be less than 1% as the FR varied from 0.083 to 2110 μW/cm2. The luminescent signal increased by 0.11% for every degree increase in temperature in the studied range of 0-55 °C. FR distribution tests were performed in different media (air or water) with the water transmittance at either 95% or 85% determined in a 1-cm path length. The FR distribution of the near-lamp region (e.g., radius<50 mm) was well determined with the nearest distance to the sleeve being less than 3 mm. Comparisons were made between the experimental data and the calculations by use of the UVCalc model. This work demonstrates that the MFSD is a novel technique that can provide in situ and real-time measurements of the FR distribution in a UV reactor.