In this work CaSO4:Eu2+ phosphor was synthesized with various molar solutions as 0.01M,0.1M,1M by coprecipitation method and the particle size effect on OSL sensitivity for CaSO4: Eu2+ samples has been studied. Synthesized phosphor was characterized by XRD, SEM, TEM and OSL measurements. Average particle size for 0.1 M and 0.01 M is found to be in the range 100nm-500nm and 80-100nm respectively with hexagonal morphology. The sample prepared with 0.1M solution shows maximum sensitivity with a slow decay of about 60 s. As the particle size reduced to 80-100nm range sample shows low OSL sensitivity with fast decay curve. For sample prepared with 1M solution, 66% reduction in OSL sensitivity is observed under identical conditions. The observed OSL in 0.1M sample compared with sensitivity of α-Al2O3: C (Landuer) and found to be 18 % using area integration method. It may find applications in radiation dosimetry using OSL techniques.