In this work, crystalline phase formation, microstructure and mechanical properties of a lithium disilicate (LS2, Li2Si2O5) glass-ceramic in the SiO2-Li2O-Al2O3-P2O5 system were investigated. A four-stage heat treatment process was used for crystallization of the glass. The effects of holding time and temperature at the final stage on the crystalline morphology of the glass-ceramic were studied. The experimental results revealed the two crystallization peaks at 672 and 839 A degrees C. At a temperature lower than 770 A degrees C and holding time of 20 min, the lithium metasilicate (Li2SiO3) phase dominates. On the other hand, when the glass was heated to a higher temperature or held for a longer time, the LS2 phase dominates and some other minor phases such as cristobalite and lithium phosphate emerge. Scanning electron microscopy and energy dispersive spectrometry revealed a large number of nanosized ZrO2 particles when the crystallization temperature was above 790 A degrees C. Vickers hardness of the LS2 glass-ceramic was about 8.1-8.4 GPa and flexural strength was in the range of 282-307 MPa. Crack deflection was observed along the LS2 cluster boundaries. The crystallization sequence was proposed to explain the observed microstructure and phases.