I applied for admission for a PhD at a university in the US in mid December. After contacting a potential advisor, he told me that he is interested in my CV. However, I didn't get any response about the status of my application. So, should I send an email to the professor asking him about the decision of my application or should I contact the university directly?
It's usually the university that handled admissions. Your professor may not have any clue which stage your application is in. So your best bet is contacting the university graduate research department, and asking for timeframes around outcome.