This study presents methods for treating a kind of nanoclay and investigates the effects of methylaluminoxane (MAO) exposure time and or dodecylamine (DDA) reflux time on in-situ polymerization of ethylene in the presence of nanoclay and examines the morphology and properties of the prepared polyethylene/clay nanocomposites. The results revealed that by increasing MAO exposure time productivity decreased. Modification of nanoclay by NH3/MAO led to formation of exfoliated structures. In treatment with NH3/DDA, the change in reflux time resulted in different structures. SEM demonstrated that the morphology of the nanocomposites strongly depended on the modification method and treatment time. Dynamic mechanical analysis indicated that the elastic modulus of the nanocomposites increased by increasing MAO exposure time. The nanocomposites treated with different MAO exposure times showed similar thermal degradation behavior. The nanocomposite modified under the condition of 24 h DDA reflux time indicated the lowest thermal decomposition temperature due to poor dispersion of nanoclay in the synthesized nanocomposite. The nanocomposites modified by NH3/MAO had higher degree of crystallinity compared to those modified by NH3/DDA, which could be attributed to the difference in dispersion level of the modified clays. A relationship between the rheological properties, weight fraction and dispersion of modified nanoclay was observed. The findings showed that the modification condition had a significant influence on the morphology and properties of the synthesized nanocomposites.