The paper reports the Bose-Einstein condensation (BEC) of over 10^5 Li2 molecules in an optical trap, starting from a spin mixture of fermionic lithium atoms. The molecules are formed through three-body recombination near a Feshbach resonance during forced evaporative cooling. The condensate eventually reaches a long-lived thermal equilibrium state. The characteristic frequency of a collective excitation mode is measured, and the magnetic-field-dependent mean field is demonstrated through controlled condensate spilling. The experiment uses a single Gaussian laser beam from an Yb:YAG laser at 1030 nm, with a waist of 23 μm. The optical trap is characterized by radial and axial oscillation frequencies of 14.5 kHz and 140 Hz, respectively, and a trap depth of 800 μK. The magnetic field tunes the scattering length, which is crucial for the formation of weakly bound dimers. At a magnetic field of 764 G, where the scattering length is positive and the binding energy is about 2 μK, the condensate is observed to contain a large number of molecules, indicating a molecular BEC. The molecular BEC is characterized by a repulsive mean-field potential, which stabilizes the condensate against collapse and decay. The mean-field properties are further investigated by applying a magnetic field gradient to tilt the vertical trapping potential, demonstrating the mean-field nature of the molecular BEC and its dependence on the magnetic field.The paper reports the Bose-Einstein condensation (BEC) of over 10^5 Li2 molecules in an optical trap, starting from a spin mixture of fermionic lithium atoms. The molecules are formed through three-body recombination near a Feshbach resonance during forced evaporative cooling. The condensate eventually reaches a long-lived thermal equilibrium state. The characteristic frequency of a collective excitation mode is measured, and the magnetic-field-dependent mean field is demonstrated through controlled condensate spilling. The experiment uses a single Gaussian laser beam from an Yb:YAG laser at 1030 nm, with a waist of 23 μm. The optical trap is characterized by radial and axial oscillation frequencies of 14.5 kHz and 140 Hz, respectively, and a trap depth of 800 μK. The magnetic field tunes the scattering length, which is crucial for the formation of weakly bound dimers. At a magnetic field of 764 G, where the scattering length is positive and the binding energy is about 2 μK, the condensate is observed to contain a large number of molecules, indicating a molecular BEC. The molecular BEC is characterized by a repulsive mean-field potential, which stabilizes the condensate against collapse and decay. The mean-field properties are further investigated by applying a magnetic field gradient to tilt the vertical trapping potential, demonstrating the mean-field nature of the molecular BEC and its dependence on the magnetic field.