Techniques of the surface modification of advanced engineering ceramics attract a growing interest. Silicon nitride based ceramics with different compositions were manufactured by sinter-HIP method. Nitrogen ion implantation has been performed by using a heavy ion cascade accelerator. Annealing investigations were made in air at different temperatures up to 1400degreesC. The oxidation effect on nitrogen ion implanted silicon nitride based ceramics produced with variable processing parameters and the newly formed crystalline phases were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and Fourier Transform Infrared spectroscopy (FTIR). Bending strength and elastic modulus were also determined. The influence of oxidation on the mechanical properties was modified by the implantation of nitrogen ions. Implantation seems to be responsible for the change of inter-granular phases and for the partial healing of the critical defects.