Much efforts have been devoted to achieve conductivity control in the ultrahigh band gap (∼6.1 eV) AlN by Si doping. The effects of Si-doping on the structural and optical properties of AlN epilayers have been investigated. X-ray diffraction studies revealed that accumulation of tensile stress in Si-doped AlN is a reason for the formation of additional edge dislocations. Photoluminescence (PL) studies revealed that the linewidths of both band-edge and impurity related transitions are directly correlated with the density of screw dislocations, Nscrew, which increases with the Si doping concentration (NSi). Furthermore, it was formulated that the band-edge (impurity) PL emission linewidth increases linearly with increasing Nscrew at a rate of ∼3.3±0.7 meV/ 108 cm-2 (26.5±4 meV/ 108 cm-2), thereby establishing PL measurement as a simple and effective method to estimate screw dislocation density in AlN epilayers.