This paper investigates the Static Noise Margin (SNM) of Ultra-Thin-Body (UTB) SOI SRAM cells operating in subthreshold region using analytical solution of Poisson's equation validated with TCAD simulations. An analytical SNM model for UTB SOI SRAM cells operating in subthreshold region is presented. Our results indicate that back-gate bias (Vbg) can mitigate the Read SNM (RSNM) variability of UTB SOI SRAM cells in the subthreshold region, and the improvement of SNM variability is more significant than superthreshold region. Increasing cell β-ratio shows limited improvement on RSNM and has no benefit on SNM variability for subthreshold operation. The UTB SOI 8T SRAM cell exhibits RSNM 2X larger than the 6T SRAM cell in subthreshold region.