Turkish citizenship

Hello, my grandfather was born in Istanbul and was a Turkish citizen until he gave up voluntarily his citizenship in in the 80's. when my father was born (1960) he was still a Turkish citizen, but he didn't register his birth (he lives outside of turkey). am I entitled to receive the Turkish citizenship as well (I was born in 1990)?

You are referring to the European concept of jus sanguinis (“law of the bloodline”).  That holds true in most of Europe but it must be uninterrupted lineage i.e. no one should have renounced the citizenship before passing it on.  This is called citizenship by descent or ancestry.

However, from what I know, Turkish citizenship by descent works slightly differently.  A child born to a Turkish father or Turkish mother is a citizen.  No mention of grand parents.

Applying this concept, your father is actually the one who should be claiming citizenship first before you become eligible - as he never registered his birth as a citizen.  If that goes through then it is your turn.

Your best bet is to consult a lawyer in Turkey on how to go about it.