Extracellular β-amyloid (Aβ) deposit is considered as one of the primary factors that induce Alzheimer's disease (AD). The effects of various environmental factors, including temperature, ionic strength, and pH, on Aβ (1-40) aggregation mechanisms were investigated in this study by spectrometry, isothermal titration calorimetry (ITC), and hydrophobic fluorescence assay. In the aggregation process, the secondary structure of Aβ (1-40) transforms to the β-sheet conformation, which could be described as a two-state model. As the temperature and ionic strength increase, the conformation of Aβ converts to the β-sheet structure with an increased rate. Results of circular dichroism monitoring demonstrate that the rate constant of nucleation is smaller than that of elongation, and the nucleation is the rate-determining step during the overall Aβ aggregation. The β-sheet structure was stabilized by hydrophobic forces, as revealed by the ITC measurements. The different structural aggregates and forming pathways could be identified and discriminated at high and low ionic strengths, resulting in distinctive fibril conformations. Furthermore, the thermodynamic analysis shows that hydrophobic interaction is the major driving force in the nucleation step. Our study provides an insight into the discriminative mechanisms of β-amyloid aggregation via kinetics and thermodynamics, especially the first reported thermodynamics information obtained by ITC.