Here's the script reference for transform.Rotate of Unity http://docs.unity3d.com/Documentation/ScriptReference/Transform.Rotate.html
Try version 3 of Rotate functions. See the given example here:
void Rotate(Vector3 axis, float angle, Space relativeTo = Space.Self);
using UnityEngine;
using System.Collections;
public class Example : MonoBehaviour {
void Update() {
transform.Rotate(Vector3.right, Time.deltaTime);
transform.Rotate(Vector3.up, Time.deltaTime, Space.World);
}
}