hi am sending in my application for a greencard but i have a petty theft expunged record. does the immigration officer need to know that?

You must disclose the offense on your any application filed with USCIS regardless of whether it was expunged or not. You will also need to provide a certificate of disposition for the arrest. Your failure to disclose the arrest can lead to serious consequences including a determination that you made a material misrepresentation. If you were convicted, you should have an attorney review the certificate of disposition to make sure you are not ineligible for a green card. You can read more about adjustment of status at http://myattorneyusa.com/adjustment-of-immigration-status.