How to make addition using textbox textchanged event in vb.net

Well yes - of course it will!
Every time you get a change in your vat textbox, you alter the value in the total amount box - which is what you use as the input for the calculation. So when you next make a change to the vat, you can only use the output from the previous calculation as the input to the next!

The proper solution (and by far the easiest) is to have separate subtotal and total boxes: you use the subtotal as the input, and output to the total. You should never try to change the inputs to your calculation!

I have a couple of suggestion for you, probably it will either fix it or help to fix it.

1) Break down calculation, so you can debug easily, rather than end up with single equation with no clue of it issue.
2) Avoid passing each time to VAL() function its an overhead. rather store it in a variable.
3) As far as i understand, you are probably losing your original value, you should have three textboxes. Lets see how:

- Take a TextBox name as txtActualAmt with a `Text` of 3500
- Take a TextBox name as txtTA with a `Text` of 10 <-- Change the value of this textbox
- Take a TextBox name as txtOutputVat with a `Text` of 3500

Now use this code:

Private Sub txtTA_TextChanged(ByVal sender As Object, ByVal e As System.EventArgs) Handles txtTA.TextChanged
    Dim str As Double = 0
    Dim actualAmount As Double = Val(txtActualAmt.Text)
    Dim TA As Double = Val(txtTA.Text)
    Dim percentageAmt As Double = Val(actualAmount * TA) / 100
    str = actualAmount + percentageAmt
    txtOutputVat.Text = str.ToString("0.00")
    str = Nothing
End Sub

Result:

- If TA = 1 , txtOutputVat.Text = 3535.00
- If TA = 10, txtOutputVat.Text = 3850.00
etc..

Notice that:

str = actualAmount + percentageAmt

shows that 'str' is the sum of actual amount and percentage on it.

Hope this is waht you were trying to do.