As an Iranian visiting scholar, I'll be living and studying in Belgium for six months. Now, I want to take part in a summer school for ten days in the USA.
Can I apply for USA visa from Belgium while I'm residing in Belgium not as a citizen?
You hold Iranian nationality and will be a temporary resident of Belgium at the time you wish to travel to the U.S. You want to know if you can apply for a visa to visit the U.S. for ten days at a U.S. embassy or consulate in Belgium.
The United States does not restrict which countries you can apply for a nonimmigrant visa from, as many other countries do. You can apply within any country you've entered legally. You would have to do so anyway, since there is no U.S. consulate in your home country.
Nonimmigrant visa applicants who are residents of Iran may apply at any U.S. embassy or consulate that provides nonimmigrant visa services, but should be aware that Farsi-speaking officers are only available at the U.S. Embassy in Yerevan, the U.S. Embassy in Ankara, and the U.S. Embassy in Abu Dhabi, and the U.S. Consulate General in Dubai.
One thing to note is that the embassies/consulates in different countries may have different processes for paying for your application and booking your appointment for your interview at the embassy or consulate. You will want to check the appropriate web page for the country you are in (e.g. if you are in Belgium you need to book your appointment and pay the application fee through a separate website run by a contractor).
To apply for a visa to visit the U.S.:
Note that because of visa reciprocity, the maximum visa validity an Iranian citizen can obtain for a B-1/B-2 visitor visa is a single entry visa valid for 90 days. If your documentation demonstrates that you can afford your visit to the U.S. and will depart at the end of your visit (and will not overstay your visa), you should have no trouble obtaining the visa.