The strength of ceramics is limited by the presence of defects. These can be introduced during the production of the material (in the bulk) or through machining of the specimens (on the surface). Therefore, to obtain representative strength measurements of the material, the machining of the specimens has to be carried out very carefully which is generally time consuming and expensive. Because the surface of ceramic components is also machined (at least partially), the strength and the reliability of components depends also on machining. In a uniaxial stress state only cracks perpendicular to the stress direction are dangerous. On the other hand, in a biaxial stress state all cracks, in any direction, are dangerous and can lead to failure. This is the reason why a biaxial test is more suitable for the investigation of the influence of surface defects on strength than a uniaxial one. In this work the influence of different surface conditions on strength is investigated by the biaxial Ball-on-Three Balls (B3B) test. A commercial silicon nitride (producer: FCT Ingenieurkeramik, Germany) was used for the analysis. It could be shown that the details of surface finishing have a strong influence on the mean strength of the material. Data scatter is also influenced if surface defects have a greater size than volume defects. If the surfaces are ground with coarse diamond grit, or if the abrasive grains are blunt, strength decreases significantly. Through a subsequent polishing the defects which were introduced during grinding can be removed to a great extent. But because of exposing new defects near the surface this leads to a great scatter of the strength values. In order to heal surface defects a special heat treatment was developed. Micro structural investigations showed that a thin glassy layer is generated on the surface of the specimens. This glassy layer fills and blocks the micro-pores and cracks at least partially (if they are not too deep). As a result the strength of the specimens increases significantly whereas the scatter of the strength data decreases. Because of the high machining costs and - in some cases – because of the geometry of the components, it is not always possible to improve the surface quality through a better finishing process. Therefore a combination of a careful machining and a heat treatment seems to be an effective way to improve strength and reliability of silicon nitride components with modest effort and costs.