C Corp tax filing is the annual process of reporting income, deductions, credits, and tax liability to the IRS. All domestic corporations must file Form 1120, regardless of whether they have taxable income. This requirement applies unless the corporation is exempt under Section 501. SDO CPA LLC Every domestic C corporation must complete this filing annually — regardless of income level, profitability, or business activity during the period. Specifically, failure to file triggers immediate penalties even for inactive corporations with zero revenue.