As of January 1, 2018, the new USA Baseball Bat Standardwas implemented. USABat Standard bats must be used in the Little League Major Baseball Division and below.
Rule 1.10 - Baseball
The bat must be a baseball bat which meets Little League specifications and standards as noted in this rule. It shall be a smooth, rounded stick and made of wood or of material and color tested and proved acceptable to Little League standards.
Little League (Majors) and below:
It shall not be more than thirty-three (33) inches in length nor more than two and one-quarter (2¼) inches in diameter. Non-wood bats shall be labeled with a BPF (bat performance factor) of 1.15 or less;
EXCEPTION: For the Little League (Majors) and below, for regular season play and Tournament, composite bats are prohibited unless approved by Little League International.