I hav varicose vein behind knee,in this place under the skin i can feel pea size lump,not protruding.vein not painful.is the lump sth serious?

Maybe : while the veins themselves can be benign clot forming inside them can be problematic. You should be checked to make sure not involving deep veins.

Behind knee lump: There are many possibilities for what you describe including a superficial clot in a varicose vein, a small lipoma or even a popliteal cyst which would be less likely. See a vein specialist for an evaluation of your veins to rule out a clot and possibly due a venous ultrasound.