Let $H$ be a rectangle with angle between two diagonal $\leq 45^{0}$. Rotation $H$ around the its center with angle $0^{0}\leq x\leq 360^{0}$ we have rectangle $H_{x}$. Find $x$ such that $[H\cap H_{x}]$ minimum, where $[S]$ is area of $S$.
1. **Understanding the Problem:**
   We are given a rectangle \( H \) with the angle between its diagonals being \(\leq 45^\circ\). We need to find the rotation angle \( x \) such that the area of the intersection \( [H \cap H_x] \) is minimized, where \( H_x \) is the rectangle \( H \) rotated by \( x \) degrees around its center.

2. **Analyzing the Geometry:**
   - The angle between the diagonals of a rectangle is always \( 90^\circ \). The condition that this angle is \(\leq 45^\circ\) implies that the rectangle is actually a square. This is because only in a square, the diagonals bisect each other at \( 90^\circ \) and the angle between any two lines drawn from the center to the vertices is \( 45^\circ \).

3. **Rotation and Intersection:**
   - When we rotate a square \( H \) by \( x \) degrees around its center, the area of intersection \( [H \cap H_x] \) will depend on the overlap of the two squares.
   - The minimum area of intersection occurs when the squares are rotated such that their sides are aligned at \( 45^\circ \) to each other. This is because at \( 45^\circ \), the overlap is minimized.

4. **Calculating the Minimum Intersection Area:**
   - When \( x = 45^\circ \), the two squares intersect in such a way that the intersection forms an octagon. The area of this octagon is less than the area of the square.
   - For any other angle \( x \) between \( 0^\circ \) and \( 360^\circ \), the area of intersection will be larger than at \( 45^\circ \).

5. **Conclusion:**
   - Therefore, the rotation angle \( x \) that minimizes the area of intersection \( [H \cap H_x] \) is \( 45^\circ \).

The final answer is \( \boxed{ x = 45^\circ } \).