Nematic-isotropic phase transition in liquid crystal elastomers (LCE), which drastically alters the macroscopic properties of the material, is attractive for both fundamental physics and applications. Here, we introduce new experimental methods based on polarized and fluorescence optical microscopy to characterize the phase transition of polydomain LCE, monitoring the order parameter and the nematic domain structure. It is demonstrated that polydomain main-chain nematic LCE exhibit an ordering transition very different from the classical first-order one, having a diffuse continuous nature. Unexpectedly, characteristic correlation length of the fading polydomain structure remains unchanged across the transition.