Imaging by MRI in diagnostic centers has got a wide application; on the other hand, its biological effects, especially its chromosomal aberrations are not completely distinguished. Researches in this area are incomplete and in some cases contradictory. thus, in 1997, the present research was carried out in Tarbiyat Modares University to determine the effects of the MRI fields on chromosomal aberration as well as cellcycle process in human peripheral lymphocytes in vitro.This is an experimental study and has been done on 30 blood samples in two sensitive phases of G◦ & G2 of cell cycle with and without the presence of DNA repair inhibitor (ara.c). Exposure conditions were magnetic field intensity: 1.5 Tesla, Rf 63.86 MHZ and exposure Time 30 & 60 minutes. The effects of MRI on the chromosomal aberrations, by the metaphase analysis and the evaluation of the effect on cell cycle via the mitotic index method were assessed.Recorded results showed that the rate of chromosomal aberration and mitotic index frequency was, respectively, as follows: in control group in G◦ 0.5% and 9.5%, in G2 0.5% and 2.3%, and in samples under MRI exposure in G◦ 0.5% and 8.5%, in G2 0.5% and 2.56%, in samples along with 100µ, ffiolara-c in G◦ 2.5% and 2.9%, in G2 19% and 0.57%, and in samples under MRI exposure and along with ara-c in G◦ 2.5% and 3.03%, in G2 19.5% and 2%. Mter assessing the results through the X2 test, no significant differences were found between control group and samples under MRI exposure, whereas there was asignificant difference between the control group and the ara-c group and also samples under MRI exposure and ara-c in the both stages of the research (P<0.05).Results obtained from this study indicated that the fields along with MRI, with the aforementioned characteristics on human peripheral lymphocytes in vitro, in neither G◦ nor G2 of cell cycle, even in the presence of DNA repair inhibitor and by doubling the normal time of exposure used in imaging centers didn't influence the frequency of chromosomal aberration and cell cycle process.Since a few researches have been done in the are a of MRI biological effects.with high intensity, especially for determining possible later effects, so such studies should be carried out constantly with higher exposure time and intensity on various species of cells.