Luminescent conjugated polymers are widely used in organic optoelectronics. The device is fabricated by spin coating the polymer solutions into thin films. It is important to understand the chain conformation in the solution phase, which is mainly determined by the solubility properties of the solutes and the solvents. The purpose of this study is focused on the aggregate structures of MDMO-PPV polymer in the solution mixing of toluene, heptanes, and decahydronaphthalene. Compared to the polymer in toluene solution, the absorption and fluorescence spectra in the mixing solutions are red-shifted, which indicates the aggregation between polymer chains. In order to identify the aggregation is inter-chain or intra-chain effect, we perform concentration dependent measurements of the fluorescence spectra down to 10-10 M. Our results suggest that inter-chain aggregation is the major influence in the concentration. Since the intra-chain aggregation is strongly influenced by polymer concentration, we carry out the experiments to identify how the inter-chain effect influences at even lower concentrations. Fluorescence correlation spectroscopy (FCS) is used to determine the particle size at 10-12M concentration, which relates directly to the aggregation size. The results show that particle size in the poor solution is larger than that in the good solution. Therefore, we conclude that the change of the fluorescence spectra is caused by the inter-chain aggregation even at the concentration to 10-12M.