Abstract: The treatment of tritiated water(HTO) produced during the operation of nuclear power plants involves the challenging process of H₂O/HTO separation, for which non-radioactive H₂O/HDO separation is usually used as a research model in laboratories. Recently, membrane distillation processes based on multi-layered graphene oxide membranes showed good performance for H₂O/HDO separation, but further improvement still need developing novel membrane materials or structures. In view of this, mono-layered graphene oxide nanoplatelets with different lateral sizes were utilized to prepare PTFE supported graphene oxide composite membranes for H₂O/HDO separation in membrane distillation:(1) two types of mono-layered graphene oxide nanoplatelets with different lateral sizes were chosen to investigate the impact of lateral size of mono-layered graphene oxide nanoplatelets on the performance of H₂O/HDO separation. The graphene oxide membrane based on GO nanoplatelets with the smaller lateral size gives the permeation flux of 0.944 L/(m²·h) and the separation factor of 1.043, and its overall separation performance is higher than that of graphene oxide membrane obtained with the larger lateral size of graphene oxide nanoplatelets; (2) mono-layered graphene oxide nanoplatelets with different lateral sizes were mixed in various mass ratios for the combination of their structural characteristics to prepare composite membranes. When the mixing mass ratio of the two sizes of graphene oxide nanoplatelets is 1:1, the resultant graphene oxide membrane exhibits permeation flux of 0.806 L/(m²·h), higher than that of the graphene oxide membrane prepared with the larger lateral size, and the separation factor of these two membranes is similar; (3) mono-layered graphene oxide nanoplatelets with the larger lateral size were etched to introduce in-plane pores. The resulted GO membrane gave enhanced H₂O/HDO separation performance. These results demonstrate that the lateral size of mono-layered graphene oxide nanoplatelets can affect the performance of corresponding membranes for the separation of H₂O/HDO in membrane distillation, and the membranes based on mono-layered graphene oxide nanoplatelets with smaller lateral size or mono-layered graphene oxide nanoplatelets with different lateral sizes mixed in a certain ratio has better performance for the separation of H₂O/HDO in membrane distillation. Moreover, the separation performance of H₂O/HDO in membrane distillation can also be improved by introducing in-plane pores to graphene oxide nanoplatelets. Therefore, this work can guide the development of novel membrane materials or structures to achieve better H₂O/HDO separation performance in membrane distillation and provide solutions for the treatment of radioactive wastewater containing tritiated water.