Longitudinal analysis of word and sentence acquisition in early language development is critically important. In this paper, we examine two issues using quantitative methodology: (1) the common features of language learning in two children, whose learning speeds were remarkably different, (2) the relationship between word and sentence acquisition. From an analysis of digital audio recordings of conversations between children and their parents, we identified the following common developmental features: (1) the ratio of lexical category used by infants was constant up to 48 months, although the children were quite different in maximal speed of lexical acquisition (56 and 30 words/month), (2) from quantitative analysis of the data, including data in several previous reports, vocabulary spurt begins during the period from fifteen to twenty months and continues for about one year, and the midpoint of the vocabulary spurt period is negatively correlated with maximal speed of lexical acquisition. (3) the use of 100 and 300 words corresponded to the appearance of two and three word sentences, respectively, (4) Mean Length of Utterance (MLU) began to increase around at the month of the vocabulary spurt, (5) both maximal speed of lexical acquisition and increasing rate of MLU of S. A. are as twice as those of S. K. It strongly suggests from these quantitative results that word acquisition process closely interacts with sentence acquisition process.