I have a knee pain when i run,but when i walk or do aerobics it's fine what could be the problem?

Knee pain: The good news is that low impact exercises doesn't cause the pain. This most likely suggests mild osteoarthritis of the knee. The best treatment is to include exercises that help to strengthen the quadriceps muscles. I would also do as it seems like you are doing which is to incorporate different exercises in your routine to include some cardio, core exercises (both for back and abs), and legs.