This study aims to increase our knowledge of Mandarin lexical tone duration in continuous Mandarin speech. Related variation factors such as the number of syllable(s) in word, the position of syllable in word, its prosodic position and speech style were also explored. Large corpora of casual and journalistic speech (total ∼1000 hours) were used. More than 90% of the words (tokens) used in spoken Mandarin are monosyllabic and disyllabic words. In casual speech, 67% of the wordtokens are monosyllabic and 30% of the word-tokens are disyllabic. In journalistic speech, however, disyllabic words (49%) are more frequently used than monosyllabic words (45%). Tone 4 is the most frequently used tone among the four lexical tones in both casual (34%) and journalistic (36%) speech. Tone 1, Tone 2 and Tone 3 have similar occurrence frequencies in causal speech. Tone 3 appears to be the least frequently used tone in journalistic speech. With regard to tone duration, the results show that Tone 2 tends to have the shortest duration in causal speech and Tone 3 appears to have the longest duration in journalistic speech. Nonetheless, the studied variation factors (number of syllable(s) in word, position of syllable in word and prosodic position) are all found to influence the duration of Mandarin lexical tones, for both causal speech and journalistic speech. Tone durations in monosyllabic words appear to be closer to those of word-final syllables than to other syllable positions in multi-syllabic words. In terms of prosodic position, tone duration tends to increase with higher prosodic level in both casual and journalistic speech. Regardless of tone nature and speech style, the longest tone duration is in phrase-final position, followed by word-final and then word-medial position. Regional variety for tone duration is explored using casual speech productions from speakers of five major cities of North and South-East China, namely Beijing, Shanghai, Wuxi, Suzhou and Nanjing.