Who's name will I write, my stepdad or biological father in N-400?

The form only asks for parent information if your parent is a US citizen. If that only applies to your stepfather, you can put down his name and make clear on the form that he is your stepfather. If your birth father is also a US citizen, you can put in his information and then add an addenda page to clarify that your stepfather is also a US citizen along with all of his other information. Due to the limitations of the Lawyers.com Forums, Alan Lee & Arthur Lee, Esqs.’ (the "Firm") participation in responding to questions posted herein does not constitute legal advice, nor legal representation of the person or entity posting a question. No Attorney/Client relationship is or shall be construed to be created hereby. The information provided herein by the Firm is general, and requires that the poster obtain specific legal advice from an attorney. The poster shall not rely upon the information provided herein as legal advice nor as the basis for making any decisions of legal consequence.