How to tell my co-advisor I am leaving for PhD? I am doing my master's and I am considering doing my PhD at a different institution. I asked my advisor for reference and he was fine with it. My co-advisor insisted on me staying. I tried to talk him multiple times to be able to leave on good terms before I started applying and he was very reluctant to me leaving. My discussions with him were not productive and every time he starts suggesting projects for me and tries to convince me to stay. Discussion gets heated when I say I want to leave my options open and ends that way. I have to say that he is a very good advisor, but that reason is not enough for me to stay.  

I decided to apply without letting him know and I did. My advisor is now asking me to let my co-advisor know that I am applying, and that I owe him that. I am sure he will be very angry and another heated argument will start again. What to do in a such a situation?
Tell your co-advisor. If he doesn't react well, take your advisor with you and have a second meeting. Maybe they can work it out colleague to colleague rather than professor to student. Withholding a good letter of recommendation because you want a promising student to stay and work with you is extremely selfish and borderline misconduct.