Proton therapy is one of the efficient methods in both control and treatment of patients with cancerous tumors. Protons, like other heavy charged particles, lose almost all their energy near the end of their range. Therefore it is possible to cover tumor?s volume with high radiation dose without any side effects in healthy surrounded tissues. In proton therapy calculations, it is important to investigate nuclear interactions by proton beams with the atomic nuclei of the stopping materials specially human body. In this project, using MCNPX 2.6.0 code, we have investigated nuclear interactions of proton beams of energies of 250, 300 and 500 MeV with using f8 tally and adding ft8 res special treatment tally in human body and have determinated both type and amount of many various isotops which are produced as a result of nuclear interactions of protons with elemental components of soft and hard tissues. Our results are compared with previous works in which semiemprical cross sections for nuclear interactions of protons have been used. In addition, it has been found that about 20% of the incident protons in these energy ranges are absorbed by the human body (via non-elastic nuclear interactions) and the rest of protons pass through the body while losing a fraction of their energy via ionization and excitation the atomic target nuclei. In this work we report results of computer simulation and calculation of nuclear interactions by protons. In MCNPX 2.6.0 code, library files of proton interaction (LA150H and ENDF70 prot) include nuclear cross section data in low energies.Also tumors are treated with protons in 60-250MeV energy ranges. So proton nuclear interactions with 60-200MeV energy with elemental components of soft and hard tissues are investigated for beams wich compeletely stop through human body. and both type and amount of many various produced isotops have been determinate. One important feature of proton therapy is that it produces a small amount of positron-emitting radioisotopes along the beam path through the non-elastic nuclear interaction of protons with target nuclei such as 12 C, 14 N and 16 O present in body tissues. These radioisotopes, mainly 11 C, 13 N and 15 O, allow imaging the therapy dose distribution using positron emission tomography (PET) technique. In this project we use MCNPX 2.6.0 simulations to predict the yield of positron emitters produced by a 250 MeV proton beam in soft tissue and our results in comparision with some previous works with different cross section data resources show fairly good agreement. Proton beams with different energies have different sharp narrow bragg peak and ranges. On the other hand, tumor treatment needs a spread out bragg peak (SOBP) to cover tumor volume completely. So it is possible to creat a flat wide bragg peak in result of using a native narrow bragg curve and adding some weighted other ones. In this project we produce a spread out bragg peak in 4 cm width using 250 MeV proton beam bragg curve and adding eight weighted other bragg curves in different energy and ranges with MCNPX code. Key words : Proton therapy, nuclear interactions, positron emitters, MCNPX, bragg curve .