We have successfully determined the internuclear distance of I-2 molecules in an alignment laser field by applying our molecular structure determination methodology to an I 2p X-ray photoelectron diffraction profile observed with femtosecond X-ray free electron laser pulses. Using this methodology, we have found that the internuclear distance of the sample I-2 molecules in an alignment Nd: YAG laser field of 6 x 10(11) W/cm(2) is elongated by from 0.18 to 0.30 angstrom "in average" relatively to the equilibrium internuclear distance of 2.666 angstrom. Thus, the present experiment constitutes a critical step towards the goal of femtosecond imaging of chemical reactions and opens a new direction for the study of ultrafast chemical reaction in the gas phase.