Quaternion.AngleAxis

public static Quaternion AngleAxis (float angle, Vector3 axis);

Description

Creates a rotation which rotates angle degrees around axis.

using UnityEngine;

public class Example : MonoBehaviour
{
    void Start()
    {
        // Sets the transforms rotation to rotate 30 degrees around the y-axis
        transform.rotation = Quaternion.AngleAxis(30, Vector3.up);
    }
}