To investigate the surface tension of the natural zeolite to be used in polymer composites, wicking method was employed on the base of Washburn's equation and that of the zeolite treated with 0.5 wt.% gamma-aminopropyltriethoxysilane (gamma-APS) was also studied and compared. The apolar component gamma(SC)(LW),W of the untreated natural zeolite was 19.22 mJ/m(2) and the polar one, gamma(SV)(LW) of the treated zeolite with 0.5 wt.% gamma-APS was 27.08 mJ/m(2) and the polar one gamma(SV)(AB) was 12.23 mJ/m(2). The value of gamma(SV)(LW) component increased, while-that of gamma(SV)(AB) component decreased with gamma-APS treatment and it was due to the increasing effect of alkyl group (hydrophobic) in gamma-APS coupled on the zeolite surface.