The thematic and information structure of scientific and technical texts are arguably different among languages. This study examines the thematic structure of scientific texts in English and Arabic to see the differences in the hierarchical organization at different thematic levels. It adopts Halliday's functional model of theme-rheme and applies it to English and Arabic scientific texts. The paper mainly investigates the three levels of theme: textual, interpersonal and experiential with the intention of discussing their translations into Arabic. It uses a corpus data of two scientific texts. The syntactic and textual elements of those texts were compared and contrasted and professional translations were provided to study the Arabic thematic structure. The data provides English and Arabic versions which allow for a comparable analysis of structure, convention and style. The study reveals that the most frequent type of themes and thematic progression is the experiential theme. It shows that those experiential themes are almost always occupying initial positions. The structure of themes has the tendency to be reproduced in the translation. However, the findings of this investigation indicate that position of themes may change as a result of translation, or changed from experiential into textual.