In this study, Sc concentration dependence of ScxAl1-xN /AlN /poly-crystalline diamond /Si surface acoustic wave (SAW) characteristics at high Sc from 23.8 to 44.3 %was investigated by fabricating one-port SAW resonator at high frequency. 3.8 GHz one-port resonator fabricated on Sc0.43Al0.57N showed an excellent performance of electro-mechanical coupling coefficient (K2) as high as 6.34% for 2nd mode Sezawa wave, which enables a wide bandwidth in high frequency applications. The temperature coefficient of frequency was approximately -40 ~ -50 ppm/deg for the device fabricated with Sc concentration of 42.9%. This is smaller value compared to conventional high K2 bulk materials such as LiNbO3. As the result, high K2 6.34% material system at higher Sc concentration of ScAlN/AlN/PCD found to be possible at high phase velocity of 7,000 m/s. Combined with the extremely high-power durability of diamond based device, high power durable wideband SAW device at high frequency can be expected.