In this paper,semiconductor laser bars were packaged on self-standing CVD diamo nd films as heatsinks to improve the thermal performance of the devices.At first,the self-standing diamond films wer e prepared by electron-assisted chemical vapor deposition (EACVD).Improving the quality of diamond films by add ing O_2at an optimized flow rate to etch non-diamond phase was adopted in deposition process,and thermal conductivity of diamond films was improved consequently.Then,electro-optical characteristics of semiconductor la sers packaged on different diamond heatsinks that were prepared at different O_2flow rates were measured,and ther mal performance of the packaged devices was analyzed.The results show that thermal conductivity of the diamond heatsink prepared at O_2flow rate of 5sccm could reach 1812.3WK－1m－1. The slope efficiency of the device is 1.30W/A,and the peak electro-optical c onversion efficiency is 60.6%. The red shift wavelength of the packaged device is 2.02n m under continuous wave of 30A,and operating temperature is reduced by 4.9K.The thermal resistance of heat transf er path of the device is reduced by 28.4%,and excellent reliability is shown.