First off, don't do it that way: Do not concatenate strings to build a SQL command. It leaves you wide open to accidental or deliberate SQL Injection attack which can destroy your entire database. Use Parametrized queries instead.
Second, the content of your text box is probably responsible for the error: if it contains spaces, or semicolons, or any other special characters SQL will get confused and you will get an error. You could enclose it in quotes, but using parametrised queries will fix the problem, and save your database from damage at the same time.
"So can u give one little example about parametrized ...
i m new about all this stuff.."
Try:
Using con As New SqlConnection(strConnect)
con.Open()
Using com As New SqlCommand("DELETE FROM custmer WHERE Customer_No=@CN", con)
com.Parameters.AddWithValue("@CN", txtCustomer_No.Text)
com.ExecuteNonQuery()
End Using
End Using