This research aims at investigating the influence of two methods implemented in primary reading and writing teaching programs--phonics-based sentence method and decoding (analysis) method--on primary school 5th grade students' reading comprehension achievement. Also, the study considers the relationship between socio-economic status and reading comprehension achievement. This causal study was carried out in four primary schools which were in the districts of Adana; Seyhan and Cukurova. Out of 20 classes in these schools, 745 fifth grade students who were chosen according to the convenience sampling method to participate in the study. The data was obtained through 351 students learned reading and writing by means of decoding method in the 2008-2009 academic year and through 394 students who learned reading and writing by phonics-based sentence method in the 2009-2010 academic year. The data was collected using a reading comprehension achievement test. In the study, the students' personal information was collected using a socio-demographic questionnaire. The findings indicated a statistically non-significant difference on reading comprehension achievement scores of students who learned reading and writing by different methods. In addition, a positive and meaningful relationship between reading comprehension achievement and socio-economic status (r = 0.33, p less than 0.01). (Contains 7 tables.)