Question: I am a beginning graduate student in Mathematics. I have a professor who always brings up the fact that my GPA is not great in my undergraduate degree even though my performance is above the norm in the institution I am enrolled at the moment, so far I got straight A's in the graduate courses that I have completed. He always tries to point out at my flaws in my undergraduate years to humiliate me in front of other professors and my colleagues. What would be a wise reply/reaction to such abuse? 

I feel bad about it cause I feel like I am harassed and I don't reply.
I am also currently taking a course with that professor.

Answer: 
Maybe you could be interested in Non Violent Communication.

From wikipedia:
<https://en.wikipedia.org/wiki/Nonviolent_Communication>

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> It is based on the idea that all human beings have the capacity for compassion and only resort to violence or behavior that harms themselves and others when they do not recognize more effective strategies for meeting needs.
>  Habits of thinking and speaking that lead to the use of violence (social, psychological and physical) are learned through culture. NVC theory supposes all human behavior stems from attempts to meet universal human needs and that these needs are never in conflict. Rather, conflict arises when strategies for meeting needs clash. NVC proposes that people identify shared needs, revealed by the thoughts and feelings that surround these needs, and collaborate to develop strategies that meet them. This creates both harmony and learning for future cooperation.
> 
> 
> 

The idea behind this is that the behavior of the professor is, in some kind of way, the expression of an unmet need. This perception can help you to externalize the problem from yourself and to get out of the mental game.

<https://www.youtube.com/watch?v=4LuPCAh9FCc>
It would appear that he is trying to shame you to make himself feel better. This is not unusual behaviour, though you would hope for better from a senior academic. One solution is to call him on it enough to stop it but not enough to make an enemy of him: 

"Yes, my GPA was indeed 3.0. That seems to have really caught your attention because you've mentioned it the last 9 times we've met."

I would not try to emphasis that you are getting great grades now. The important bit is to call him on his GPA-focus, particularly if there are other people around. Do it in a manner which suggests that you are genuinely curious about why he is mentioning it, rather than letting him know that it irks you.