The most radiotherapy techniques used are intensity modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT). Multi leaf collimator (MLC) has a major rule in IMRT and VMAT technique, the accuracy of MLC depends on many factors. In linear accelerators a logfile created during treatment delivery. The logfile contains information of the planed and actual delivered leaf speed and position of MLC. This work aims to study the effect of MLC positional error on the radiobiological parameters of the radiotherapy treatment planning and how the logfile can be useful to detect the error which can be effect on the quality of treatment planning. There are two major radiobiological parameters can be used for plan evaluation: Tumor Control Probability (TCP) and Normal Tissue Complication Probability (NTCP). In this study the TCP and NTCP parameters were measured using MATLAB program as biological evaluation tools of treatment plans before and after MLC error applied from logfile analysis. 10 Head and Neck VMAT cases were selected for this study. The results showed an increase in the TCP and NTCP values once the MLC error has been corrected in the TPS from the data of the logfile. the average TCP value is 82.64% for the original plan and 84.96% for the plan after MLC modification, while for the NTCP, there are some variations in the results from normal organs to another. In conclusion the logfile has important role to discover the error of MLC position which may affect on the radiobiological parameters of planning evaluation.