The exploitation of shale gas has increasingly become important as a sustainable energy source but also has environmental issues including the production of wastewater with high total dissolved solids (TDS). Membrane distillation (MD) is a promising treatment option for such wastewater but may suffer from serious membrane fouling. Accordingly, this study examined the effect of pretreatment and operating conditions on the fouling behaviors of MD membranes. Hollow fiber membranes made of polyvinylidene fluoride (PVDF), polyethylene (PE), and polypropylene (PP) were compared in a laboratory-scale direct contact MD (DCMD) system. Real shale gas wastewater from oil and gas operations in Texas, United States was used. Results showed that the flux reduction ratio ranges from 13.6% to 27.7% to achieve 50% recovery ratio without pretreatment. Application of the pretreatments including FMX, flocculation - sedimentation (FS), and flocculation - sedimentation - microfiltradon (FSMF) was found to be effective to retard MD flux decline. The characteristics of the fouling layers on the membrane surfaces were examined using scanning electron microscopy (SEM) and excitation emission matrix (EEM) analysis to understand the fouling mechanisms.