The diameter of Czochralski (Cz) sapphire crystals is 50 mm. The sapphire substrates were lapped by using diamond powders and polished by chemical mechanical polishing(CMP) method using alkali slurry with SiO2 abrasive. After obtaining the smooth surfaces,the chemical etching experiments were processed by using fused KOH and NaOH etchants at different temperature for different times. The dislocation was observed by means of optical microscope and scanning electron microscope. The clear and stable contrast images of sample etching pits were observed. On the whole,the dislocation density is about 104-105 cm-2. Comparing the results under the conditions of different etchants,temperatures and times during the etching proceeding,it was found that the optimal condition for dislocation displaying is etching 15 min with fused KOH at 290 ℃. At the same time,the formation of the etch pits and the reducing method of dislocation density were also discussed.