Background: To better understand the intricate science of reading development, both cognitive and affective factors must be taken into consideration. This developmental study aims to enrich the literature by exploring how reading motivation -- an affective factor -- contributes to reading comprehension in Chinese elementary schoolers, beyond well-accepted cognitive-based reading skills, such as decoding and vocabulary. Methods: We used a cross-sectional design with 420 native Mandarin-Chinese-speaking students (Grade 2: N = 95; Grade 3: N = 216; Grade 4: N = 109), with age-appropriate materials to measure their decoding skills, vocabulary knowledge, reading motivation, and reading comprehension. Three sets of hierarchical regression analyses were run for the three grade samples to examine the unique contributions of reading motivation to reading comprehension while controlling children's demographic profiles, such as age and gender, and two cognitive-based reading skills. Results: We replicated previous findings: children's decoding and vocabulary are robust predictors of reading achievement across Grades 2 to 4. Importantly, reading motivation contributed uniquely to reading comprehension in all three grades and explained an increasing amount of the variances in reading comprehension from Grades 2 to 4. Intrinsic reading motivation was critical in Grade 3, while extrinsic reading motivation became important in Grade 4. Conclusions: Taken together, our findings reveal the complexity of reading development and advance the existing cognitive-based reading theories by including affective dimensions.