We present a fully differential measurement on above-threshold ionization of diatomic (CO) and linear triatomic ( ${\mathrm{CO}}_2$ ) molecules in strong femtosecond laser fields ( $25\mathrm{fs}$ , $795\mathrm{nm}$ ) at a laser intensity of $\sim 8\times {10}^{13}\mathrm{W}/{\mathrm{cm}}^2$ . The photoelectron longitudinal momentum distribution of CO in the laser polarization direction shows a broader distribution near zero momentum in the laser polarization plane with a shoulder around $0.38\text{a.u.}$ , whereas it shows a clear maximum for ${\mathrm{CO}}_2$ . The relative photoelectron yields near zero longitudinal momentum are less dependent on the laser intensity. The difference may result from the molecular structure. Even though molecular targets have different binding energy and outer orbitals, the near-threshold photoelectrons have similar angular distribution, and the dominant angular momentum of the near-threshold photoelectron is $L=4$ (ng Rydberg state) for both targets.