Can I marry my fathers 1st cousin brother.. means my grandfathers younger brothers son?
I want to know that weather there is any chance of genitical disorder for tha foetus?

Well, your father's first cousins brother would be your father's first cousin as well.

This is okay in many areas of the world and genetic disorders are a risk no matter who you marry. With that said, certain ethnicity's are at an increase for some genetic disorders.

It would also depend if marrying a blood relative is allowed in your culture or country/region.