RNA structures are intrinsically dynamic, and the dynamics are important for interactions and recognition. We developed a strategy using femtosecond dynamics to quantitatively investigate the heterogeneous nature of RNA structures. The different conformations that exist in an RNA molecule are resolved based on their distinct temporal behaviors of the fluorescent decays for the incorporated fluorophore on ultrafast time scales. Using this strategy, we have probed the GNRA tetraloop motif and revealed alternative structures that have not been directly observed by other techniques. A model is proposed for the coexisting multiple conformations and the conformational dynamics for this motif.