At what temperature CaCO3 decomposed when p=1 atm ?

If you want the exact value, googling calcium carbonate should give you the answer. But if you carried out an experiment, you'll need to use the results that you got. Or if you were given some values, you'll need them.

Yeap I saw something... but I think is more complicated because there is a reverse chemical reaction with the CO2 as a product..

Well, it can happen. Can you tell us what it's all about now?

From what you just said, it's an experiment. Were you told to make weightings? Use a thermometer perhaps? Heat to constant mass? And all the routine procedures concerning gravimetry?