Lipopolysaccharide plays an important role in the outer membrane of most Gram-negative bacterium, because it consists of the innate immune system for these bacteria. Lpt system is the only pathway to transport lipopolysaccharide in most Gram-negative bacteria. Among them, the outer membrane protein LptD is involved in the last step of lipopolysaccharide transportation, so it is regarded as a “vital gate”. LptD is involved in a variety of biological processes, including organic solvent tolerance, hydrophobic antibiotic resistance and membrane permeability. Recent studies have shown that the most important function of LptD is its role in the transportation of lipopolysaccharide. To help further understand LptD and reveal the Lpt mechanism in other gram-negative bacteria, we described the structure and functions of LptD in some Gram-negative bacteria.