Based on the system of modality in Systemic Functional Linguistics, this paper compares the Chinese and English speeches on science and technology with a self-built corpus, focusing on the different types, orientations and values of modality. It is found that the modality types are similar in Chinese and English speeches, but obvious differences are shown in their choice of modality orientation and value. Chinese speeches are characterized by more implicit objective modal realizations whereas the English ones are featured by frequent explicit or implicit subjective modality. Moreover, Chinese speeches prefer modal expressions with middle or low values while modality value makes little difference in English speeches. The reasons for the similarities and differences are related to social cultural and situational contexts of the two languages, which contributes to the interpersonal meaning in speeches.