Oil-in-water (o/w) Pickering emulsions stabilized by polyacrylamide (PAM) and poly(acrylamide-co-acrylic acid) nano-sized crosslinked polymeric particles (nanogels) under various conditions are described herein. Nanogels with different crosslinking and charge degree were synthesized through suspension polymerization and characterized at various salinities and pH using dynamic light scattering (DLS) and scanning electron microscopy (SEM). The PAM nanogel-stabilized o/w Pickering emulsions were quickly demulsified in alkaline solutions whereas they showed markedly stability in brines and under acidic conditions. A small amount of nanogels were sufficient for the formation of Pickering emulsions. The average diameter of oil droplets was no longer decreasing with nanogel concentration increment above 1000 mg/L. In addition, the average diameter of oil drops was independent of sonication period in the range of 15-240 s. Interestingly, the string-like structures of nanogel aggregations were found in the aromatic hydrocarbons/water Pickering emulsions, which significantly enhanced the emulsion stability. Compared to the crude oil, the corresponding Pickering emulsions showed excellent flowability at various shear rates. The crude oil/water Pickering emulsions could be broken using alkali as a trigger, which suggests the potential utility for the recovery of crude oil.