One basic problematic aspect in x-ray diffraction phase analysis is microabsorption effect which may arise from the sizeof the crystallite phases. Complication of the problem may intensify in sintered ceramic materials where milling of thesamples is not simple. We report the Rietveld x-ray diffraction phase analysis of MgO-􀁡-Al2O3 powder mixtures withphase content ratio of 1:1 by weight and MgO-Y2O3 sintered ceramic composites with Y2O3 contents of 10%, 20% and30% by weight. The mixtures were pre-sintered at 1000􀂰C for 2 hours and then milled while the composites weresintered at 1550􀂰C for 3 hours. The phase composition analysis was done using Rietica, a non-commercial Rietveldmethod-based software. Relative and absolute phase compositions were examined and results showed that there was asignificant amount of phase composition bias resulted from the examination. For the powder mixture, milling can reducemicroabsorption effect and hence the calculation bias. For the ceramic composite where milling is almost impossible,additional of Y2O3 caused smaller crystallite size of MgO, so that composition bias is smaller in composites with higherY2O3 content. A mathematical model is proposed to provide more acceptable phase composition results.