We define the complex powers of a densely defined operator $A$ whose resolvent exists in a suitable region of the complex plane. Generally, this region is strictly contained in an angle and there exists $\alpha\in[0,\infty)$ such that the resolvent of $A$ is bounded by $O((1+|\lambda|)^\alpha)$ there. We prove that for some particular choices of a fractional number $b$, the negative of the fractional power $(-A)^b$ is the c.i.g. of an analytic semigroup of growth order $r>0$.