The Fully Depleted Silicon-On-Insulator (FD SOI) MOSFETs have been already proved as a favorable CMOS technology node beyond 100 nm and the concept of source modification has been adopted in order to improve the electrical controllability of the device. The one of the major challenges in the design of FD SOI MOS device is the appropriate selection of front and back oxide (BOX) thickness. As the selection of front oxide, with proper thickness and material, is a pathway to avoid off-state gate tunneling to conquer various off-state leakages. While, the buried oxide (BOX) layer is dominant to overcome short channel effects and to reduce drain electric field penetration towards the SOI/BOX interface. This paper demonstrates the impact of front and back oxide (BOX) thickness variations on performance of modified source FD SOI MOSFET at 50 nm gate length. The impact of front and back oxide (BOX) thickness variations have been analytically evaluated by using twodimensional surface potential and drain current model, and verified against the simulated results. It has been verified that the device exhibits more uniform electric field in the channel. That assures low off-state leakage current and reduced shortchannel effects in the device. The I_on to I_off ratio of the device is found to be 10¹⁰ and hence the device is suitable for low power digital applications. The device has also been designed and simulated using TCAD device simulator.