This paper presents an experimental work on the compressive strength of earth bricks in particular the influence of additives. In total 21 different test series are carried out with different composition of earth, cement, gypsum, hemp and flax fibers. The earth material is characterized by geotechnical laboratory tests. The test results indicated that the compressive strength is highly dependent on the density of the bricks. The fibers hemp and flax have rather low impact on the compressive strength of earth bricks, but they have strong influence on the breaking behavior. Cement and gypsum as additive, as they seem to reduce the binding force of the clay minerals, lead to a highly decreased strength.