To simplify the expression \( X^3 - 9X^2 + 27X - 27X^3 - 9X^2 + 27X - 27 \), we can first combine like terms.

Let's break it down:

1. Identify the coefficients for each power of \( X \):

- For \( X^3 \): \( X^3 - 27X^3 = -26X^3 \)
- For \( X^2 \): \( -9X^2 - 9X^2 = -18X^2 \)
- For \( X \): \( 27X + 27X = 54X \)
- For the constant term: \( -27 \)

2. Combine these results:

\[
-26X^3 - 18X^2 + 54X - 27
\]

Therefore, the expression simplifies to:

\[
-26X^3 - 18X^2 + 54X - 27
\]