If you purchased an annuity that provides income in retirement, the portion of the payment that represents your principal is tax-free; the rest is taxable. The insurance company that sold you the annuity is required to tell you what is taxable.
Different rules apply if you bought the annuity with pretax funds (such as from a traditional IRA). In that case, 100% of your payment will be taxed as ordinary income. In addition, be aware that you’ll have to pay any taxes that you owe on the annuity at your ordinary income-tax rate, not the preferable capital gains rate.
Source: Financial Planning for Women