Hi Mouna.

1. are you sure that your amplified product is correct in the positive control?  At least check it's length on a gel.  

2. Assuming your desired product is correct, is the product for the NTC the same as for the positive control?  If you are not sure, check it on a gel.  

3. If positive control and NTC products are different then likely you have a dimer problem in the NTC. In which case you should either redesign primers, try higher melt temp, lower primer concentration, or acquire data at a higher temperature.  My favourite of these is redesigning primers.  

4. If positive control and NTC products are the same you have a contamination problem.  If this is the case then I suggest:

• ensure you separate your PCR reaction set up area, reagents and equipment from the thermocycling and PCR product handling area reagents and equipment.
• use RNAse Away or similar decontaminating solution for your benches.  
• don't sterilize consumables in autoclaves that take waste
• there is usually no need to autoclave anything
• make up water, Taq and primers into many small aliquots.Avoids contamination of large quantities, and if you suspect contamination it's no big deal to discard the aliquot.
• change tips for every single pipetting step, unless the previous tube is completely clean and empty.
• buy your water: molecular or PCR grade 

Quick question: what sort of Ct values are you getting (both for your test samples and for your no template controls)? 

It is relatively rare for random contamination to give you signal comparable with actual test signal, so this suggests that either you have really quite severe contamination (if you NTC Cts are low, but this seems unlikely) or simply have very low signal (i.e. high Ct values, more vulnerable to noise/contamination/pimer dimers).

So knowing what sort of Ct values you're getting would be helpful.

i have tried to seperate my pre and post PCr area, and decontaminated every thing as possible,and still have this amplification in some of the NTC.  i still have not run them on gel to see which type is.

for the CT, i have ct in the test about 25,5, 24,8 and  for NTC of this gene is about 26,8 , another gene i have 20  and 28 FOR NTC and the tesy is about 20, 27, 29.

Excellent.  Then I strongly suggest you run them on a gel. Also if you could post some pictures of amplification and melt curves that may be helpful.