Is the conjugation map always an isomorphism? Given a group $G$ with $a\in G$ fixed, define $\phi: G \to G$ by $\phi(x) = axa^{-1},  x \in G$, I am wondering when $\phi$ is an isomorphism. I think that it is always an isomorphism because it is one to one, and the order of the domain ($|G|$) and the order of the range are equal. Am I wrong in thinking this?


Given a group $G$ with $a\in G$ fixed, define $\phi: G \to G$ by $\phi(x) = axa^{-1},  x \in G$, prove that $\phi$ is always an isomorphism. 

You wrote:


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*I think that it is always an isomorphism because it is one to one, and the order of the domain ($|G|$) and the order of the range are equal. Am I wrong in thinking this?


That gives you that $\phi$ is bijective if $G$ is finite. Otherwise, you can prove that $\phi$ is bijective by establishing that $\phi^{-1}$ exists. 
To prove that it is always an isomorphism: you also need to show that $\phi$ is a homomorphism: that is, that $\phi$ satisfies the following property:
$$\phi(xy) = \phi(x)\phi(y), \forall x, y \in G.$$ The proof is fairly straightforward: 
Let $x, y$ be any elements in $G$. Then:
$$\phi(x)\phi(y) = (axa^{-1})(aya^{-1}) =  ax(a^{-1}a)ya^{-1} = a(xy)a^{-1} = \phi(xy)$$
