Dear Djouaher Zo,

I hope this helps a bit. I show the principle using a 4th order polynomial:

sols = NSolve[Sum[(RandomReal[] - 0.5) x^i, {i, 0, 4}] == 0, x]; 

In this case I get:

sols
(*{{x -> -0.946069}, {x -> -0.171399 - 0.607033 I}, {x -> -0.171399 + 0.607033 I}, {x -> 0.748391}}*)

Now I can choose the real and positive solutions:

Select[Cases[Chop[x /. sols], _Real], # > 0 &]
{0.748391}

You can do this in one command, e.g. just Select:

Select[Chop[x /. sols], (Head[#] == Real && Re[#] > 0) &]

Cheers,

Marco