in case of a company income tax payable shall be higher of the two:-
a) tax on total income computed as per normal provisions of the act
b) 18% of book profits. ( w.e.f. AY 2012-12 18.5% of the book profits)

where in case of a company the income tax payable on the total income computed under the IT Act,1961 in respect of previous year relevant to the assessment year is less than 18% of its book profits, such book profit shall be deemed to be the total income of the assessee and the tax payable by the assessee on such total income (book profits) shall be the amount of the income tax at the rate of 18%.

In case of a company, Tax payable is higher of either 18.54% of book profits ( book profits to be calculated as per method prescribed u/s 115JB ) or 30.9 % of Income calculated as per income tax act. In some cases due to some adjustments like depreciation or adjustments u/s 43B of income tax act or some other adjustments, tax @ 18.54% of book profits comes higher than the tax as per normal provisions of income tax act, and in such cases, tax becomes payable as per provisions of section 115 JB, which is called MAT i.e, minimum alternate tax.