Abstract: For obtaining ash samples with different contents of CaO, various quantities of CaO are added in the raw uraniferrous lignite, the mixtures are burned and the ashes are leached in the same conditions. The test results show that the variance of degrees of sintering does not depend on CaO contents in ash; however, the uranium extraction is increased as CaO contents are increased in ash.Since CaO content in the ash is increased, the ash tends to be more basic and can be easily attacked by acid, i.e. the uranium lock-up in ash can be easily released to the advantage of uranium extraction. The data from tests also indicate that the constants a and b in the formula η=(d/ad -b) ×100% are decreased with increase of CaO content in ash. In addition, if leaching conditionsare intensified,a,b will also be decreased. Thus it can be seen that a and b are in close relation to the solubility of the ash itself.