The aim of this study was to investigate factors that hinder critical thinking in the Iranian High Schools’ curriculum. Both methods of qualitative and quantitative research were adopted. In the qualitative part a grounded theory approach was developed while in the quantitative section of the methodology a survey method was adopted. The population under study in the qualitative part was curriculum development experts (educationalists) who were working in the Education Ministry and responsible for the curriculum of high schools in Iran, through biased sampling 18 experts (educationalists) were chosen as the cases for study. The research instrument for the data collection was deep interview. The population under study in the quantitative part was all of the curriculum development experts (educationalists) and the teachers of high school in Tehran. With regard to the Morgan table, the sample size was 97 for the experts and 375 for the teachers. The random sampling was used for choosing the experts and multi-stage cluster sampling was used for the teachers. The research instrument was a 24-item, five-option likert scale questionnaire designed by the researchers. In order to analyze the data the frequency table, the mean indices, standard deviation, one sample t-test and the Friedman test were used.The qualitative results indicate that, in the stage of open conceptualization-coding (the primary development of concepts), 27primary concepts were discovered and extracted, and in the stage of open conceptualization-coding (the development of main issues) according to the analysis of concepts and issues, 7main issues were extracted. They were policies and prophecies of the educational system, the social-cultural conditions of the society, teacher, content, the teaching method, goals, the critical thinking skills, training good citizens and good experts. In the pivotal coding (determination of conditional, procedural, and consequential issues), the conditional issues (policies and prophecies of the educational system, the social-cultural conditions of the society), procedural issues (teacher, content, goals, and the teaching method), the consequential issues (training good citizens and experts) and finally the ground theory are explained.The quantitative results indicate that developing critical thinking in high school faces some barriers such as: low attention paid to the skills of analysis, synthesis, ambiguity tolerance, risk taking, and accepting the opponent views and etc.