RichTextBox richTextBox1 = new RichTextBox(); richTextBox1.BackColor = Color.Yellow; richTextBox1.SelectionStart = 0;(starting point of text selected ) richTextBox1.SelectionLength = 4;( the number of characters selected ) richTextBox1.SelectionFont.Bold = true; richTextBox1.SelectionStart = 9;(starting point of text selected ) richTextBox1.SelectionLength = 5;( the number of characters selected ) richTextBox1.SelectionFont.Bold = true;