The existing procedure used at the domestic pressurized water reactor plants for estimation of the critical boron concentration at startup following any reactor trips has many weak points from the point of view of accuracy, and its effectiveness is very doubtful. In this thesis, efforts are concentrated especially on the establishment of new estimation schemes for the control rod worth and the boron worth, both of which have been the main causes of big error. The effect of axial power shape change on the control rod worth is newly taken into account using some correlations based on the simulation data with relevant variables. And the boron calculation is improved through reflecting the power dependency of differential boron worth. Some suggestions are also made to improve the accuracy and applicability of the estimation for the other reactivity parameters such as power defect, zero power temperature defect, and poison worth changes. As a result, it is shown that the estimation of the critical boron concentration by use of the improved model can reduce the error size enough to be applied to practical use.