Surface modification of meta-aramid fibers was performed by phosphoric acid treatment. Surface morphology and element composition of untreated and treated fibers were analyzed by scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS). Effects of surface modification on the mechanical strength of aramid papers made from meta-aramid fibers and fibrid were investigated. Surface modification conditions were optimized by response surface analysis (RSA). Results show that phosphoric acid treatment increases the surface roughness and oxygen content of aramid fibers. They improve the interface bonding strength between aramid fibers and fibrid, which improves the tensile strength of aramid papers. The results of response surface analysis indicate that the tensile strength of aramid papers increases by 47.5% and reaches the maximum when meta-aramid fibers are treated with 21.1wt% phosphoric acid solution at $39.3^{circ}C$ for 36.7 min.