Aim: Due to the rapid development of technology and growing knowledge of radiobiology, radiotherapy is getting stronger everyday as a radical method of cancer treatment.Dose delivery to the target volume and sparing the surrounding normal tissues becoming easier by means of Intensity Modulated Radiotherapy (IMRT), Image Guided Radiotherapy (IGRT), Tomotherapy and so on.Can the quantitative radiobiological models of calculating tumour control probability (TCP) and normal tissue complication probability (NTCP) be able to predict the precise outcome of any radiotherapy treatment plan? The ratio alpha/beta is a measure of a tissue sensitivity to fractionation.It has been suggested in many papers that the alpha/beta value for prostate cancer cells could have a value as low as 1 to as high as 5.This is in sharp contrast to the accepted value of an alpha/beta of 10 for most tumours.How sensitive these values are in radiotherapy planning?Materials and Methods: Two 3D-CR and an IMRT treatment plans were developed with an initial prescription dose of 60 Gy in 2 Gy/fraction to prostate.Then the prescription dose was gradually increased from 60 Gy to 70 Gy in 1 Gy steps.TCP and NTCP are calculated.Sensitivity of TCP and Complication free tumour control probability (P+) to the different values of alpha/beta ratio is investigated for various prescription doses planned to be delivered in either a fixed number of fractions (Ⅰ) or in a fixed dose per fraction (Ⅱ) in each of the three different treatment plans.Results: Lower alpha/beta values give the higher TCP when prescription doses are planned in 30 fractions.In those cases, the dose/fraction increased with prescription dose.TCP are not significantly sensitive to the alpha/beta values when prescription dose are planned in 2 Gy/fraction.This agrees with previous work that if indeed the alpha/beta is lower than previously thought then increasing the dose/fraction would be radiobiologically advantageous.Conclusions: The large dose/fraction causes the high NTCE So even though a high TCP can be achieved with a large dose/fraction, due to large NTCP.overall P+ become low for high prescription dose.High dose/fraction and high alpha/beta value result in comparatively smaller P+ and IMRT plans resulted in the highest P+, mainly due to the decrease in NTCP.So, if alpha/beta is really lower than expected, better tumour control can be achieved by increasing dose/fraction but decreasing the number of fractions.