If I make "after tax" contributions to a 401K, why can't I withdrawal some of that money without the IRS 10% penalty?

[macuser_22]

Why do you think that you cannot?    Of course, withdrawing after-tax money only depends on the 401(k) plan rules.

[dmertz]

Be aware, though, that distributions from the after-tax sub-account in your 401(k) (which includes any pre-tax investment gains attributable to the after-tax contributions) which may be distributed separately from the rest of your 401(k) are distributed as a proportionate mix of pre-tax and after-tax amounts.  That means that if there are investment gains, at least a portion of the distribution will be taxable and subject to the early-distribution penalty.