<xsl:variable name="ID"><xsl:value-of select="./@id" /></xsl:variable>
the result of this is
T123, Y445, G666, Y5454

IF I want to have display like

T123
Y445
G666
Y5454

How can i do that?

I am using version 1.0

Firstly

<xsl:variable name="ID"><xsl:value-of select="./@id" /></xsl:variable>

is a ludicrously inefficient way of saying

<xsl:variable name="ID" select="./@id" />

Why create a result tree fragment when you only want a string, especially as it makes your code twice the length?

Presumably therefore you have an @id attribute containing the string

T123, Y445, G666, Y5454

You can convert this to your required output using

translate($ID, ', ', '#xa;')

However if your output is HTML then generating newlines isn't enough to have each string displayed on a newline, you need to generate <br/> elements. For that you need to tokenize the string - go to www.exslt.org and find the str:tokenize() template. It's built into some processors but if not there's an XSLT implementation you can copy into your stylesheet.

Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference