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Microplastics have raised many concerns because of their potential negative impacts on the environment. Identifying the sources of microplastics release to the environment is a challenging subject. Synthetic textiles, such as machine woven carpets, are highly capable to propagate and emit microplastics, especially fibers. While the carpet cleaning service has experienced a fast growth due to urbanization and its impact on life style, there is a lack of information on the number and the size of fibers, which are released during the washing process. In this study, we characterized the microplastic fibers in the wastewater of two carpet-washing workshops in the cities of Ahwaz and Sari in Iran. Three replicates of 10 L-samples were taken from the wastewater of washing and drying stages. All samples were passed through sieves of 500, 300 and 37 μm. The residues were washed with 1 L distilled water and poured into clean glass bottles. Then, the samples were passed through 25 μm filter paper. The remained materials on the filter were examined using a stereo microscope. Shapiro-Wilk and Levin tests were applied to test the normality and homogeneity of data. One-way ANOVA test was used to investigate the differences in size of microplastic fibers and independent t-test to determine the difference between the total number of the released microplastic fibers and the ones at each stage in the cities. More than 3097 and 1824 microplastic fibers per square meter of carpet (equal to 81 and 48 microplastic fibers per liter of wastewater) were counted in the workshops in Ahwaz and Sari, respectively. The shares of microplastic fibers in the size of ≥500, 300-500 and 37-300 μm were 18.4%, 24.6%, 57% in Ahwaz and 14.4%, 28.8%, 56.8% in Sari, respectively. The number of released microplastic fibers per liter of wastewater of carpet washing workshops is much higher than the number of fibers in the raw wastewater entering Sari WWTP, which was 4.9-12 microplastic fibers per liter. The washing of machine woven carpets is an important emission source of microplastic fibers especially with the size of less than 300 micron. The number of released microplastic fibers depends on the type of washing and drying practices. It is expected its emission load will sharply increase in future due to the booming growth in demand for these kinds of reasonably priced floor covers.