Yousef Nadarkhani is currently serving a ten year sentence after being accused of crimes against the state linked to the running of churches.
He's written to authorities saying his refusal to eat is a "cry of a father, unjustly imprisoned".
The pastor took extreme action after hearing his 15 year old son Yoel had been told he could not return to school as he had not completed Islamic studies. Religious minorities are normally exempt from this class.
His 17 year old son Daniel has also been denied a school report card which is needed to enroll onto a higher education course.
They're being targeted because authorities say children of Christian converts should be treated as Muslims.
Nadarkhani says this shouldn't apply because he never practiced Islam prior to his conversion.
He says his hunger strike is for his own children but also other Christian youngsters who are targeted.
Religious freedom charity Christian Solidarity Worldwide (CSW) is campaigning on his behalf.
Chief executive Mervyn Thomas said: "CSW is deeply concerned that Pastor Nadarkhani's children have been denied access to education on account of their religious beliefs.
"Even more disturbing are indications that other second generation Christian children are experiencing similar difficulties.
"This directly contravenes a Supreme Leader's fatwa permitting children raised in Christian homes to access appropriate education.
"We call on the Iranian authorities to rectify this situation by allowing Yoel Nadarkhani and other children facing similar discrimination to return to school, and providing Daniel Nadarkhani with the documentation necessary to enable him to enroll in university. We also continue to call for the immediate and unconditional release of Pastor Nadarkhani and all others from religious minority communities who are currently imprisoned for their faith."
Nadarkhani was originally arrested in 2009 but was acquitted of apostasy. He's been in and out of prison ever since.
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