I donated 18 acres of land I inherited to my niece .

[stan2nation]

I donated 18 acres of land I inherited to my neice.  Is that deductible.

[Hal_Al]

No. There is nothing for either you or your niece to report on your income tax return. It is not deductible for you or income for her.

The  rule, for a gift, is that the recipient's basis is the giver's basis. Her cost basis, for when she sells it in the future,  is therefore the fair market value of the land on the date of death of the person you inherited it from.

[Hal_Al]

If the value of the land, on the date of the gift (not the decedent's DOD) was more than $14,000, then you may be liable to file a gift tax return.
"Gift Tax" is somewhat of a misnomer.  Even though a gift tax return may be required, very few people ever actually pay federal gift tax. The purpose of the gift tax return is usually only to document a reduction in the allowable estate tax exemption.
See <a rel="nofollow" target="_blank" href="https://turbotax.intuit.com/tax-tools/tax-tips/Tax-Planning-and-Checklists/The-Gift-Tax-Made-Simple/...>