Summary: In this paper we determine the sets of spherical roots, real roots, isolated complex roots, pure imaginary quaternion roots and roots in \(\mathbb R + \mathbb R j\) and \(\mathbb R + \mathbb R k\) of a quaternion polynomial \(Q(t)\) by corresponding these sets to the sets of real or complex roots of some real or complex polynomials determined by \(Q(t)\). Thus, the counting and classifying methods for such polynomials can be used for the counting and classifying of the aforementioned roots of quaternion polynomials.