The purpose of this study is to figure out the effect of pressurized grouting on diameter enlargement and pullout resistance of the compression ground anchor installed in decomposed residual soils. The laboratory chamber tests were carried out for 3-types of soils which are abundant in Korean peninsula. These tests simulate the real construction process of a ground anchor with pressurized grouting. Experimental results showed that grout permeation effect into the ground through porous media has to be taken into account when estimating the anchor diameter, if the permeability of the ground is larger than 10-3 cm/sec. A series of in-situ anchor pullout tests was executed in 3 different sites which represent 3 types of residual soils used in laboratory chamber tests. Results of in-situ tests showed that the effect of the pressurized grouting is more prominent in a softer ground with smaller SPT-N value. Based on experimental results, a new equation to estimate the pullout resistance as a function of the SPT-N value was proposed which might be applicable to decomposed residual soils.