Practice: An Inquiry-Based Mathematics Activities Module (IBMAM) was developed in accordance with the developmental characteristics, interests, needs, and expectations of preschoolers. The aim of the study was to determine the effect of the IBMAM on preschoolers' numbers and operations skills. A quasi-experimental method, which was a pretest-posttest design with control group, was used. The students of a kindergarten were randomly assigned to three groups: experimental, control, and placebo. Measurements were made on the three groups before and after the IBMAM. The study took place in the academic year of 2016-2017, and the sample consisted of 57 students (19 participants in each group) of a kindergarten in Sincan, Ankara, affiliated to the Ministry of National Education. Interviews were conducted with 18 kindergartens in the same district for the needs analysis. The kindergarten was the study field of choice due to its sufficient number of students and low socioeconomic status and to the students' willingness to participate in the study. Math activities were performed in three classes with children 60-72 months of age. For 6 weeks, the experimental group performed the 30-activity IBMAM, the control group performed no activities, and the placebo group performed Turkish reading activities unrelated to mathematics (TRAUM). Data were collected using the Test of Early Mathematics Ability (TEMA-3). In the current study, 3x3 (three groups: experiment, control, placebo groups with 3 measurements: pretest, posttest, follow up test) experimental mixed design (factorial-split-plot) ANOVA was performed to determine the effect of IBMAM on preschoolers' math skills. Research Findings: Results show that the IBMAM had a positive and lasting effect on preschoolers' number and operations skills. IBMAM provided the experimental group participants with the opportunity to be engaged in small-group activities in which they were able to have math experiences and inquire about them. This process allowed them to enhance math concepts, learn new ones, and learned in meaningful ways in which they used their math skills. They structured the concepts and skills that they had learned by inquiring, researching, and experiencing, which promoted their curiosity and interest in learning.