This study aims to identify the crucial prosodic factor for native speakers' accentedness judgement of L2 pronunciation. Prosodic features are known to have more impact on the accentedness of L2 learners' pronunciation than segmental features do. Among prosodic features, timing and pitch are looked at in this study as major prosodic factors that affect native speakers' accentedness judgement of L2 pronunciation. To examine the relative importance of timing and pitch, two types of speech samples - natural speech and prosody-modified speech - were used, as these two types of speech compensate each other's shortcomings: Natural speech contains genuine speech errors, whereas in synthesised speech, the number and degree of errors are rigidly controlled. In two experiments, native Japanese listeners assessed the accentedness of these stimuli and the results were compared. Both experiments obtained the same result: that timing is more important than pitch in improving the naturalness of L2 Japanese speech.