In this work,we investigate the influence of defect concentration of the diamond substrates on the performance of hydrogen-terminated diamond field-effect transistors by Raman spectra,pulsed Ⅰ-Ⅴ characteristics analysis,and radio frequency performances measurements.It is found that a sample with higher defect concentration shows larger drain-lag effect and lower large-signal output power density.Defects in the diamond act as traps in the carrier transport and have a considerable influence on the large-signal output power density of diamond field-effect transistors.This work should be helpful for further performance improvement of the microwave power diamond devices.