Microplastics (MP), defined as plastic debris smaller than 5 mm, are increasing in the ocean with the globally increased consumption of plastic products. MP can be ingested by filter feeders,  accumulated in their organs, and thus can negatively affect their growth and survivorship. However, it is still unknown under which conditions the negative impacts of MP become serious. We investigated the interactive effects of MP pollution and heat stress on the blue mussel Mytilus trossulus by an indoor tank experiment. The blue mussel was kept individually with different combination of MP density (0, 2, 20, 200 [mg/l]) and temperature (17, 20, 23[°C]) for 12 weeks. Filtration rate, faeces production, pseudofaeces production, respiration rate, and Body Condition Index (relative weight of soft tissue per unit shell weight) were measured and compared at the end of the experiments. Interactive effect was detected in the filtration rate, pseudofaeces production, respiration rate, and BCI. However,  the trends and direction of interactive effect were different among different variables, suggesting the heat stress did not always amplify MP effect. This study indicated that the interactive effect of MP and heat would seriously impact the blue mussel  in warming and more polluted oceans in the near future.