Features of tonal co-articulation in Mandarin speech are studied with a focus on how the word boundary location affects the results. Although there are several previous works investigating how the prosodic features of syllables are affected by the surrounding syllables, most of them selected nonsense syllable sequences as speech material without specific consideration on the word boundary. In the present study, however, a comparison is given on the tonal coarticulation between intra-word and inter-word situations. The speech material is designed in that, in each pair of sentences, the target disyllables share exactly the same tonal context but differ in the position regarding to the word boundary: the boundary locating at the initial of the target or locating at the middle. Mean and range of F0 values are adopted as prosodic features of each syllable, and mean F0’s differences between the second and the first syllables of the target are calculated and compared for the sentence pairs. Analysis on all of the 16 disyllabic tone combinations shows the effect of word boundary location on the tone co-articulation is different depending on the tone combinations, especially when the target disyllables include a Tone 2 syllable.