  In this paper, two semi-analytical solutions of force free fields
(\citeauthor{low90}, \citeyear{low90}) have been used to test two nonlinear
force-free extrapolation methods. One is the boundary integral equation (BIE)
method developed by \citeauthor{yan00} (\citeyear{yan00}), and another is the
approximate vertical integration (AVI) method developed by \citeauthor{son06}
(\citeyear{son06}). Some improvements for the AVI method have been taken to
avoid the singular points in the process of calculation. It is found that the
correlation coefficients between the first semi-analytical field and
extrapolated field by BIE, and also that by improved AVI, are greater than
$90\%$ below a height 10 of the $64 \times 64$ lower boundary. While for the
second semi-analytical field, these correlation coefficients are greater than
$80\%$ below the same relative height. Although the differences between the
semi-analytical solutions and the extrapolated fields exist for both BIE and
AVI methods, these two methods can give reliable results for the height of
about 15$\%$ of the extent of the lower boundary.
