The relation between bulk microdefect (BMD) and mechanical strength of $P/P^-$ epitaxial silicon wafers (Epitaxial wafer) as a function of nitrogen concentrations was studied. After 2 step anneal$(800^{circ}C/4hrs+1000^{circ}C/16hrs)$, BMD was not observed in nitrogen undoped epitaxial silicon wafer while BMD existed and increased up to $3.83 imes10^5;ea/cm^2$ by addition of $1.04 imes10^{14};atoms/cm^3$ nitrogen doping. The slip occurred for nitrogen undoped and low level nitrogen doped epitaxial wafers. However, there was no slip occurrence above $7.37 imes10^{13};atoms/cm^3$ nitrogen doped epitaxial wafer. Mechanical strength was improved from 40 to 57 MPa as nitrogen concentrations were increased. Therefore, the nitrogen doping in silicon wafer plays an important role to improve BMD density, slip occurrence and mechanical strength of the epitaxial silicon wafers.