CVD diamond film is known as the most promising materials for semiconductor device because it has many good electrical properties. To apply diamond film which have excellent semiconducting characteristics for semiconducting device, it have to overcome many difficulties such as depositing, doping, contact, patterning. To pattern the diamond film, we must apply dry etching with $O_2$ plasma because it has wet inertness. In the present investigation, it has been studied the characteristics of etching by ECR PECVD and patterning of diamond film by selective area etching after it has been deposited with Hot-Filament CVD. To study characteristics of etching in diamond film, it glow discharge $O_2$ ECR plasma at conditions which the pressure is 0.1mtorr, the flow rate is 150sccm and microwave is 1.5kw. The characteristics of etching in diamond film have been changed by the pressure conditions. At low pressure grain boundary and bulk also have been etched but at high pressure condition, grain boundary only has been etched. Commonly etching rate in grain boundary is more faster than bulk because the non-diamond, the etching rate is fast, exist in the grain boundary. It is proved by Raman spectrum with etching time that $1333cm^{-1}$ diamond peak is invaried peak intensity with etching time but $1550cm^{-1}$ non diamond peak is decreased peak intensity with etching time. After the diamond film is all removed by $O_2$ plasma, it is investigated by AES and XPS analysis that SiC growing as bufffer layer when diamond film is deposited, is not removed by $O_2$ plasma etching and oxide is growing over SiC. In the ECR $H_2$ plasma, it is not carried out the etching of diamond film because the chemical poteintial in the H2 plasma is positive. To pattern the diamond film, it is carried out by selectice area etching by Cr mask lithography and it is seen istrophic ethching characteristic when the diamond film is etched with ECR $O_2$ plasma.