1.not to mention：在列举的几项后再加一项,前后是并列关系,多为肯定句中使用.相当于as well as,如：
Pollution has a negative effect on the health of everyone living in the city,not to mention the damage to the environment.
It's too far to walk,not to mention the fact that it'll probably be closed by now anyway.
2.let alone：多用于否定句中,后加的一项是递进关系,是较之前的更加不可能的.目的是强调前一项不可能,如：
The baby can't even sit up yet,let alone walk.
当然这只是大致区别,个人觉得二者区别其实是相当小的.非常感谢不客气