Thank Brenden! my webpart is a form. As your answer, I could not re-use the form control (/CMSFormControls/Captcha/Recaptcha.ascx) for my webpart and I have to use
< cms:RecapthcaControl > then re-use some code of the form control, right?
@Hozefa, your link is posted before Kentico 8 release and now I am using Kentico 8, so I asked to re-use new form control "reCaptcha" in Kentico 8 for my page.