In this paper, the Self Heating Effect (SHE) is investigated in silicon on insulator (SOI) nanowire MOSFETs. A comprehensive study of SHE in Nanowire FET (NWFET) was implemented by Technology Computer-Aided Design (TCAD) simulation. Through analysis of on-current (I_on), thermal resistance (R_TH), and heat flux, the DC characteristics of SHE were investigated according to the increase of buried oxide thickness (Tbox). The results indicate that leakage current is not changed according to Tbox so that it is not necessary to increase T_box. In conclusion, in case that T_box was optimized by 1 nm, temperature and R_TH were reduced by 152 K and 54.5% respectively. Also, I_on was increased by 5.84% maintaining leakage current compared with Tbox of 20 nm.