In content-based instruction (CBI) programmes, students learn content subjects through a second language (L2), so that they can incidentally learn the language. To further facilitate students' L2 development, the collaboration between L2 and content subject teachers has been strongly advocated. Previous research on such kind of collaboration mainly focused on how it may be realised, and on identifying potential obstacles, while its effectiveness remains under-explored. This paper reports the findings of a small-scale project conducted in a Hong Kong secondary school in which CBI is practised. Teachers of English, Integrated Humanities and Science collaborated in the design and teaching of several units for Grade 8 students. With data collected from teacher meetings, lesson observations, and students' performance on the pre- and post-tests, this study evaluated "whether" and "how" cross-curricular collaboration contributed to students' L2 learning. The findings show that students in classes where cross-curricular collaboration was practised demonstrated a better understanding of the targeted grammatical items. This could be attributed to enhanced teachers awareness of students' needs, a more suitable curriculum and changes in teachers' pedagogical foci in lessons. This study thus suggests the potentials of cross-curricular teacher collaboration and provides important implications for further enhancing the effectiveness of CBI.