As the advent of advanced process technology such as 65nm and below, the designs become more and more sensitive to the variation of manufacturing process. Though the complicated design rules can guarantee process margin for the most layout environments, some layouts that pass the DRC still have narrow process windows. An effective layout optimization approach based on Litho Friendly Design (LFD), one of Mentor Graphics' products, was introduced to enhance design layout manufacturability. Additional to process window models and production proven Optical Proximity Correction (OPC) recipes, the LFD design kits are also generated and needed, which with the kits and rules people should guarantee no process window issues in a design if the design passes the check of these rules via the kits. Lastly, a real 65nm product Metal layer was applied full chip OPC and post-OPC checks to process variation. Some narrow process window layouts were detected and identified, then optimized for larger process window based on the advices provided by LFD. Both simulation and in-line data showed that the DOFs were improved after the layout optimization without changing the area, timing and power of the original design.