The influence of reading ability on cross-language transfer in Mandarin-speaking ninth graders was explored. Each participant's native language (L1-Mandarin Chinese) and second language (L2-English) were assessed. Although the relationship between L1 and L2 reading ability has been discussed in many previous studies, few studies have examined this relationship among L2 readers whose L1 is sharply different from their L2, who are at the junior-high-school age range, and who are learning English in a setting where English is not used in daily communication (e.g., English as a foreign language). To investigate the role of L1 reading competence in the language reading ability transfer, a reformed public examination, called the Basic Competency Test (BCT), was applied in this study. The 30,000 Taiwanese ninth grade participants, randomly selected from the pool of the national examination involved in a consecutive six-year period, were considered as a whole and then disaggregated into six groups based on the year they took the BCT. A preliminary analysis was about reliability coefficients of twelve examinations (six in Mandarin Chinese reading, and six in English reading) used in the present study. Scores from both Mandarin Chinese and English reading comprehension tests were subjected to descriptive, correlational, and regressional analyses. Both correlation and regression analyses revealed congruent results that provided support for the positive influence of Mandarin Chinese reading competence on English reading ability, that is, L2 reading ability is dependent on L1 reading competence. The finding supported the Linguistic Interdependence Hypothesis. In addition, participants' gender and school district also played statistically significant roles to affect the cross-language transfer of reading ability, whereas the length of time in English exposure had no statistically significant effect on the language reading ability transfer. Thus, cross-language transfer in reading ability was influenced by learners' L1 reading competence, gender, and school district. This suggested educational policy makers in Taiwan that boost native-Chinese speaking students' Mandarin knowledge help support the development of English reading ability. Apparently, if students' L1 reading abilities can be built up more soundly, their L2 reading ability should be easier to acquire.