This dataset is an excel file relating to the impact of atmospheric CO2 on grass (Brachypodium distachyon) silicon (Si) accumulation and the consequences for insect herbivore (Helicoverpa armigera). This study mainly focused on the hypothesis that reduced CO2 can increase Si in a grass and vice versa under elevated CO2. Moreover, we hypothesised that the variation in Si accumulation under extreme CO2 conditions can affect insect herbivore differently. In our present study, we found that reduced CO2 increased foliar Si accumulation, which was associated with decreased herbivore performance relative to elevated CO2.