should i ground arduino ??

I controll an analog circuit with arduino uno.The analog circuit is in a breadboard.Should i ground the arduino in the breadboard too?

All grounds must be connected together. Ground defines 0 volts.

i use external supply for my circuit ,not arduino supply.Is ti dangerous for arduino to ground them all at the breadboard ??

Arduino is NOT a self-powered microcntroller ,it is either powered by the PC or the battery which are both external power supplies, THERE IS NO ANY DANGER , THAT'S HOW IT SHOULD BE DONE. GROUNDING IT TO THE BREADBOARD(blue line) AND SUPPLYING IT WITH POWER (red line).

TO clirify this ,i supply the arduino from PC.I supply my circuit with 12V (supplier) because i have OP-AMPS .Arduino reads voltages from circuit.In this case is obligated to ground arduino at breadboard for reading correct voltages ?

Yes. All grounds must be connected together.