The effects of miR-148a in regulating the expression of TGFβ2 and SMAD2 in MNNG-initiated gastric cancer rats and the mechanism of action in GC cells were determined. Effects of miR-148a on the proliferation, migration, and invasion of GC cell lines were demonstrated. We used Wistar rats, Balb/c nude mice, and GC cell lines. Rats were treated with MNNG to establish a GC rat model. Levels of miR-148a, TGFα, TGFβ2, SMAD2, SMAD3, and SMAD4 were tested in gastric tissues from different groups. In GC cell lines, we constructed and transfected a primary miR-148a plasmid to determine the expression patterns of TGFβ2, SMAD2, and SMAD4. A luciferase activity assay was used to monitor the effects of miR-148a on the TGFβ2- and SMAD2-3'UTRs. We identified nude mouse models bearing BGC-823-miR-148a or BGC-823-vector cells. Tumor volumes were detected, and TGFβ2, SMAD2 expression levels were determined in tumor tissues. The in vivo study demonstrated an increase in the mRNA and protein levels of TGFβ2, SMAD2, and SMAD4 in the MNNG-treated group compared with the control group. However, there were no differences in the mRNA and protein levels in either TGFα or SMAD3. The in vitro study demonstrated that overexpression of miR-148a reduced TGFβ2 and SMAD2 significantly in GC cells. The results of the luciferase activity assay showed that miR-148a could bind to the 3'UTRs of TGFβ2 and SMAD2 and inhibited their activity. Overexpression of miR-148a inhibited proliferation, migration, and invasion significantly in GC cell lines. In vivo, tumor volume of BGC-823-miR-148a was smaller than that of BGC-823-vector. Overall, miR-148a inhibited the proliferation, migration, invasion, and expression of TGFβ2 and SMAD2 in GC cells. It was concluded that miR-148a might play an important role in gastric cancer, and is a potential candidate for GC treatment.