Microplastics (MPs) pollution has become a global challenge due to their persistent properties and harm to nature and human health. Observation of MPs pollution, especially in marine environments, is a critical concern in environmental analysis. In this work, we validate the method for detecting the presence of MPs in seawater by using fluorescence microscopy with Nile Red dye combined with Attenuated Total Reflectance Fourier-transform infrared spectroscopy (ATC-FTIR). The specificity and accuracy of this method were evaluated by analyzing the standard spiked samples. Five standard MPs and chitin samples used in this study were polyamide (PA), polyethylene (PE), polyethylene terephthalate (PET), poly(methyl methacrylate) (PMMA), and polyvinyl chloride (PVC) with a diameter from 5 to 300µm. The obtained recovery was 88.3-96.7%, with the related standard deviation (RSD%) in the range of 4.6-11.0%. This method was applied to determine microplastics in 27 seawater samples at three locations in Can Gio Sea, Ho Chi Minh City. The results showed the pollution level of each type of MP was significantly varied, with PE and PET being the highest, while there was almost no PMMA in seawater samples.