Most likely cause is a leaking cylinder head gasket. Your mechanic can do a chemical test at the radiator for that.
You didn't say what the symptom is that makes you think the engine is overheating. It can't get hot enough to overheat in just a few minutes. If you see bubbles in the coolant reservoir or the dash gauge goes to "hot" real quickly, check the coolant in the reservoir. If a head gasket is leaking there won't be any steam associated with the coolant and it won't be real hot in the reservoir yet.