Polyaniline (PANI) nanomaterials were synthesized by simplified template-free method with ammonium persulfate (APS) as an oxidant. The strong dependence of morphology of polyaniline on the initial pH of the reaction solution has been found. PANI morphology transformed from nanosheets to nanotubes with the decreasing initial pH from 6.70~6.50 to 5.30~5.00, and nanotubes changed further to irregular granules with the decreasing pH lower than 2.26. Polyaniline nanosheets were highly crystalline (2θ=6.2°) but poor in conductivity (5.7×10-5 S/cm) and electroactivity due to low doping level. Though polyaniline nanotubes were poorly crystalline, it had a relative high conductivity (3.4×10-2 S/cm) and maintained the electroactivity in acidic and neutral media which had potential applications in biomedical fields such as tissue engineering and biosensor.