This study focused on the effect of ion implantation on the mechanical properties of silicon nitride based ceramics. The examined materials contained 4% Al2O3, 6% Y2O3 and, in some cases, graphite or black carbon additives. Fully densified bodies were manufactured by two-step hot isostatic pressing, in other experiments pressureless sintering gave porous ceramics. Nitrogen or carbon ions with 190 keV or 350 keV were implanted, a part of samples obtained laser treatments for comparison. Measurements of elastic modulus, four point and three point bending strength were carried out at room temperature on sintered samples, partially sintered samples and sintered and oxidised samples. Carbon or nitrogen implantation influenced the strength of partially sintered samples only if the density was over 80%. Both the implantation of carbon or nitrogen ions and the laser treatment affected the bend strength of sintered samples, enlarging the power introduced its value first increased than decreased. Implantation slightly decreased the strength of samples oxidised at 1400 °C. Comparison of different mechanical characteristics may support a better understanding of phenomena during ion beam implantation or laser irradiation.