We report an experimental and theoretical study of maximum modal gain of p-doped 13 μm InAs/GaAs quantum dot （QD） lasers. The maximum modal gain of the QD laser with five stacks of QDs is as high as 175 cm-1, which is the same as that of the undoped laser with identical structures. The expression of the maximum modal gain is derived and it is indicated that p-doping has no effect to the maximum modal gain.We theoretically calculated the maximum modal gain of the QD lasers and the result is in a good agreement with the experimental data. Furthermore, QDs with lower height or smaller aspect ratio are beneficial to achieving a greater maximum modal gain that leads to lower threshold current density and higher differential modal gain, which is good for the application of p-doped 13 μm InAs/GaAs QD lasers in optical communications systems.