Sometimes i feel pain in my knee joint. is it reduction of cartilage? happens sometimes when i run too much.

Knee pain: There are many causes of knee pain. Cartilage, meniscus, ligaments, patella etc.. Given that the pain seems to be intermittent and is only noted sometimes when you run its probably either muscle or ligament. If you are concerned then see your doctor for an evaluation.

Unlikely: Based on your age, the more likely scenario is patellofemoral syndrome, which will often respond to anti-inflammatory medication and focused physical therapy. If the pain persists, you might consider seeing your pcp or orthopaedist for a diagnosis.