Three kinds of perovskite nanoparticles encapsulated with different chain lengths of alkylammonium, (CH3NH3)(x)(CH3(CH2)(3)NH3)((1-x)) PbBr3 (NP-C-4), (CH3NH3)(x)(CH3(CH2)(7)NH3)((1-x))PbBr3 (NP-C-8), and (CH3NH3)x(CH3(CH2)(11)NH3)((1-x))PbBr3 (NP-C-12), are successfully prepared. X-ray powder diffraction experiments demonstrate that these three nanoparticles are all pure cubic phase. However, the compositions of these three nanoparticles are significantly different, as revealed by steady-state absorption spectra. NP-C-4 mainly consists of 2D perovskite with m (number of unit cell layers) = 1 and 3D perovskite. Instead, NP-C-8 and NP-C-12 are mainly composed of 2D perovskite with m = 3, 4, and 5. Time-resolved fluorescence spectra and femtosecond transient absorption spectra suggest the presence of energy transfer from 2D perovskite to 3D perovskite in these three nanoparticles. More importantly, the energy-transfer rate gradually decreases from NP-C-4 to NP-C-12. This result suggests that the composition of perovskite nanoparticles and their corresponding photophysical properties can be controlled by the chain length of alkylammonium. This provides a new insight for preparing novel perovskite nanoparticles for special applications.