if we check dc circuit through a tester its glow. why ?

if indeed you do have one of the NEON type testers you will see two rods inside a glass bulb. With AC, both will tend to glow. Actually they are taking turns glowing at either 50 or 60 times per second, depending upon where in the world you live. For DC, only one electrode will glow. It will indicate the NEGATIVE lead. These units usually require somewhere around 55 to 60 volts to light up. Inside the housing, there is a resistor to limit the amount of current the bulb can draw.