LiFePO4 and Li0.99Nb0.01FePO4 with olivine structure were synthesized by an in situ synthesis technique. Their electrochemical behaviors in Li2SO4 aqueous electrolytes were investigated by cyclic voltammetry (CV). It shows that both compounds undergo lithium ion extraction and intercalation upon oxidation and reverse reduction at the safe potential window without causing the kinetic electrolysis of water. For LiFePO4, only one pair of symmetrical redox peaks, which are associated with the Li+ ion extraction/insertion upon the oxidation/reduction of Fe2+/Fe3+ redox couple, appears on its CV curves at both low (0.10–1.0mVs−1) and medium (5.0–50mVs−1) scan rate ranges. For Li0.99Nb0.01FePO4, various electrochemical behaviors were observed at different scan rate ranges. Two pairs of redox peaks, which are broad and sharp, respectively, appeared at the low scan rate range, but the broad peak pair disappeared at the medium scan rate range. Further study found that the reactions happening at the sharp peak pair are independent of the reactions occurring at the broad one. Various scan rate experiments revealed a linear relationship between the peak current and the square root of scan rate for all peak pairs, indicating that the Li+ deintercalation/intercalation processes occurred in both compounds are diffusion-controlled. The corresponding diffusion coefficients were calculated in the range of 10−11–10−12 cm2 s−1.