Little League® International has assembled an online resource page dedicated to baseball bat information, that includes the latest bat information, current Little League Baseball rules and regulations governing bats, definition of terms, the moratorium on the use of composite bats, and a series of frequently asked questions, with answers and licensed bat lists.
http://www.littleleague.org/learn/equipment/baseballbatinfo.htm

Please note that as of January 1, 2018, the new USA Baseball Bat Standard will be implemented. Little League-approved baseball bats that are approved for use for the 2017 season will no longer be acceptable for use in any Little League game or activity starting on January 1, 2018. For more information on the USABat standard and a complete list of bats approved through the USABat Standard, visit usabat.com.

Rule 1.10
The bat must be a baseball bat which meets the USA Baseball Bat standard (USABat) as adopted by Little League. It shall be a smooth, rounded stick, and made of wood or of material and color tested and proved acceptable to the USA Baseball Bat standard (USABat).