The high-speed optical communications and small devices were required for development of 5G and datacenter and low-loss and thin flexible printed circuit boards are also desired. The conductor losses of MSLs are known to be increase with decrease of their thickness and authors have been proposed a new low loss MSL structure with slits on its ground metal.In this report, the transmission losses of the low-profile slit MSL were simulated and its structure was optimized. It was shown that the attenuation constant of the optimized slit MSL with its thickness of 25µm is equal to that of standard MSL with 50µm thickness by measurement.