Answer
$\theta=\frac{\pi}{3}$
Work Step by Step
$y=\sqrt {3}x $
Putting $y=r\sin\theta$ and $x=r\cos\theta$, we have
$r\sin\theta=\sqrt {3}r\cos\theta$
$\implies \frac{\sin\theta}{\cos\theta}=\sqrt 3$
Or $\tan\theta=\sqrt 3$
$\implies \theta=\arctan (\sqrt 3)$
Or $\theta=\frac{\pi}{3}$