Usually, strain and temperature coefficients of Brillouin frequency shift (BFS) of optical fibers result from experimental measurements. For the first time, theoretical strain dependence of BFS is analyzed depending on fiber properties. Based on a FEM-2D modeling, the strain coefficient Cε is determined considering geometrical profile, doping composition and drawing conditions. Theoretical results showed great accordance with measurements for different types of single-mode fibers. The Cε coefficient is evaluated with a relative uncertainty better than 8%. Significant Cε magnitude variation has been observed depending on doping profile. This paves the way towards major improvement of strain optical fiber sensors.