Move gameobject at constant speed

I am new to this engine. So I need help in this basic stuff. I read the other similar question but didn’t get satisfactory answers.

Basically I am developing 2D game using tk2d plugin. I want to move my ball constantly on screen.

 float ballMagnitude = rigidbody.velocity.magnitude;
                if (ballMagnitude < VELOCITY)
                {
                    Vector3 ballVelocity = new Vector3(VELOCITY, VELOCITY, 0f);
                    rigidbody.velocity = ballVelocity;
                }

But this code creates problem when ball direction change. So I need help in this stuff.
If you want more detail then I am ready to provide. Thanks for your intention.

After some what research I have following code :

if (!isPaused)
            {
                float ballMagnitude = rigidbody.velocity.magnitude;
                print("ball magnitude : " + ballMagnitude);
                if (ballMagnitude < SPEED)
                {
                    Vector3 ballVelocity = new Vector3(VELOCITY * (Mathf.Abs(rigidbody.velocity.x) / rigidbody.velocity.x), VELOCITY * (Mathf.Abs(rigidbody.velocity.y) / rigidbody.velocity.y), 0f);
                    rigidbody.velocity = ballVelocity;
                }
            }

            if (Input.GetMouseButtonDown(0) && isPaused)
            {
                Vector3 ballVelocity = new Vector3(VELOCITY, VELOCITY, 0f);
                rigidbody.velocity = ballVelocity;
                isPaused = false;
            }

Using this code, the object moves constantly but it show some jerks in its movement. How to reduce this one?

As I understand from the code you have posted, you always apply velocity in the same direction. Maybe you need to get the direction vector and then apply velocity in that direction.