A primary adduct of MgCl2. 3. 3EtOH (A0) was prepared by melt quenching and then it was further dealcoholated by thermal and chemical methods. In thermal dealcoholation، two different support samples with MgCl2. 2. 7EtOH (A1) and MgCl2. 1. 8EtOH (A2) formulas were prepared using a controlled temperature-pressure program. XRD spectra of A0، A1 and A2 supports were similar، except with the peaks at 2θ = 8. 9 and 9. 7 which decreased by moving from A0 to A2. After preparation of support samples، final catalysts were prepared by reacting support samples with TiCl4 in the presence of diisobutylphthalate as internal electron donor and examined in slurry phase propylene polymerization using TEA as cocatalyst and diphenyldimethoxysilane as external electron donor. The results showed that in the catalysts prepared using thermal dealcoholated supports، surface area and Ti content increase by moving from A0 to A2 (decreasing ethanol content of the support). In this respect، the variation in dealcoholation level، due to different dealcoholation time، influenced the catalytic activity considerably and maximum activity was observed at A1 sample and then by further dealcoholation the catalyst activity was dropped. Also، chemical dealcoholation was carried out using 1 M triethylaluminum (TEA) at 20 and 80˚C، and the prepared supports were designated as A3 and A4، respectively. In chemical dealcoholation samples، support structure was totally destroyed and no peak was observed at XRD spectra. The final catalysts were prepared likewise، and examined in slurry phase polymerization of propylene using TEA as cocatalyst. By destruction of support structure، Ti content was decreased significantly and hence the catalyst activity was dropped compared to the catalysts prepared by thermal dealcoholation route.